There are Jewish extremists, of at least three types. The first is the Haredi Jews, also called the "Ultra-Orthodox". While once intimately tied to the fabric of Eastern Europe specifically and Jewish life more generally, over the course of the late 19th and especially 20th centuries they have begun to form their own yeshivahs and distinct communities. Today, in North America and Israel and a handful of European cities (London, Antwerp, etc.), they have largely separate communities and are marked by largely separate dress--the so-called "Black Hats". There's actually a pretty wide range of Haredi Jews, some of which, like Chabad-Lubavitch, have become quite engaged with the world, and some, like Satmar, Skver, and Tosh, have set up their own isolated villages away from other settlements (see Kiryas Joel, NY; New Square, NY; Kiryas Tosh, Quebec). In Israel, you can see that, too (B'nei Brach is the biggest city, but there's also many others--including even a separate Chabad village, Kfar Chabad). These groups are generally not violent (if that's what you mean by extremist), and most of the examples of violence have come in the past twenty years (and thus beyond discussion here), usually around either leadership fights or people wanting stricter observance (often enough, the former is justified by the later). These types of fights are more common in Hasidic rather than non-Hasidic "Lithuanian" communities (there are also Sephardi Haredi communities, especially in Israel, but that's another issue). One that is more than twenty years old is the fight for leadership in the Satmar community after its second Rebbe, Yoel Teitelbaum, died in 1979. His nephew Moshe took over but some stayed loyal to Yoel's widow. You can this article from the Village Voice which mostly describes the present issues, but also mentions some of the past ones:

"Every day you got up in the morning, you wondered who got slashed tires, who got windows broken, who got stoned?" says Ben Friedman, a dissident of the era who says his tires were slashed three times. "Every day: What happened last night?"

More clearly violent are the religious settlers in Israel today. My favorite book on them, though it's quite old now, is Ian Lustic's 1988 For the Land and the Lord. Now, not every West Bank or Gaza settler is religious or violent, but they are disproportionately religious and are associated with much of the anti-Arab violence in the country. I won't get into what's gone on in the last 20 years, but rulings by fringe rabbis (and less fringe rabbis) of what is permissible and not permissible to do to a Palestinian Arab have gotten far more extreme. They tend to believe that the Israeli state is the beginning of "redemption" (as opposed to "exile") and that things like the settlements in all of Eretz Israel (the Land of Israel, which is larger than the territory of the State of Israel) will help bring about the start of the messianic era. The military victories in 1947-9 and especially in 1967 (when Gaza and the West Bank and especially the Old City of Jerusalem were conquered) are all but proof of this holly mission. Therefore, for many of the more extreme segments, any "land for peace" deal is a betrayal of God and the Jewish people--this is precisely the sort of logic that led to the assassination of Israeli Prime Minister and Nobel Peace Prize winner Yitzhak Rabin in 1995 (19 years ago, but close enough to the twenty year rule I hope) by a Jewish extremist named Yigal Amir, who was explicitly upset by (and in his mind justified) by the Oslo Peace Process. Amir is condemned by the majority of Jewish and Israeli society, but it's not too hard to find people who support him "in principle".

The last group I'd like to point out is the Kahane-ists. Meir Kahane started as a popular Orthodox/Conservadox rabbi in New York (Bob Dylan and Arlo Guthrie both knew him in this period), but first founded the Jewish Defense League (a paramilitary "self-defense" organization) in America and then the political party Kach in Israel, which was banned in 1988 for being "racist" and "anti-democratic" (the only Jewish party to have been so banned in Israel). He was sentenced as early as 1971 on terrorism related charges for manufacturing explosives. His early violence was against Soviet targets in America as a protest for the Soviet Governments refusal to grant Soviet Jews exist visas so that they could emigrate to Israel. In Israel, he became more concentrated on Arab rather than Soviet targets. Though he was assassinated in 1990, his followers are still numerous in Israel today. It's not rare to see "Kahane Chai" (Kahane lives!), his iconic Star of David with a raised fist, or hear the slogan "Kahane Was Right". The most famous act of violence associated with Kahane, however, came after he died. On Purim (a Jewish holiday associated with the defense of the Jews against violent antisemitic persecution) 1994, Baruch Goldstein, a well respected American-born doctor and follower of Kahane, entered the Cave of the Patriarchs (where, according to Jewish and Muslim tradition, Adam & Eve, Abraham & Sarah, Isaac & Rebecca, Jacob & Leah, are all buried) in Hebron and opened fire with automatic weapons. He killed 29 people and wounded 125 more and was only stopped when the crowd subdued and then beat him to death. His tomb is in Kiryat Arbah, a settlement right outside Hebron (which is almost certainly the holiest Jewish place in the West Bank, and the one that Jewish extremists are least willing to consider giving up). Not coincidentally, it's right across from a "Meir Kahane Park". Here's a picture of his gravestone, the text of which reads (using Wikipedia's translation; Hebrew Jewish acronyms are translated with dashes separating the words):

Here-is-buried The Holy The physician Mr. Barukh Kappel Goldstein Blessed-be-the-memory-of-Tzadik-and-Holy one, May-the-L-rd-avenge-his-blood Son of Mr. Israel, who, in contrast is still alive, and should continue to a long and good life, Amen Seventh generation of Rebbe The Old "Master of the Tanya" Rabbi Shneur Zalman of Liadi Gave his life for the nation of Israel, its Torah and its land "Clean of hands and pure of heart" Born 5th of Tevet, 5717 Murdered over making the Lord holy 14th of Adar, Purim 5754 May-his-soul-be-listed-among-the-living.

The language here (thing like "may the Lord avenge his blood") makes it clear that he is considered a holy martyr for the Jewish people, not a mass murderer. While their peak was clearly before they were first banned in 1988, when Kahanist parties have run in Israeli elections, they generally get thousands of votes (though have not managed to pass Israel's 2% electoral threshold on their own--some former Kahanists have managed to be elected on other tickets).

It would be even easier to come up with a list of extremist Christians; Jews worldwide are homogenous enough to just happen to break down nicely into these three easily identifiable extremist groups (there's a good article on it in one of the volumes of Martin Marty's Fundamentalism Project). Now, keep in mind today there are roughly of fourteen million Jews and about one and a half billion Muslims (meaning, for every one hundred Muslims in the world, there's slightly less than one Jew). Meanwhile, there are over two billion Christians. To classify the extremist of Christian groups, even just in the 20th century, would be more difficult as Christian, especially Protestant, organization allows for many small, dispersed groups. But you have to remember that things like Prohibition came directly out of Protestant groups. The Klu Klux Klan justified itself as a Christian organization. As White Supremacists have become less popular, many of the groups have become more explicitly Christian (see the "Christian Identity" movement). The Oklahoma City bombers were at least loosely tied to Protestant Christian Identity groups. Similarly, Eric Rudolph bombed abortion clinics, gay establishments, and the 1996 Summer Olympics in Atlanta (just outside the twenty year rule, but again I hope alright). He was motivated by Catholicism and possibly a loose affiliation with Christian Identity groups (he denies the influence of the latter). The Olympic Bombing alone killed one and wounded 111 others. He was not the only religiously inspired person to murder or to try to murder people involved with abortion clinics. And those are only recent example from the United States off the top of my head. It would not be hard to add more.

Now, there are a good questions about why Muslim extremists have gotten more power than their counterparts in other religions, but it's not like there is a lack of Christian or Jewish extremists. It's certainly worth at least noting that, in most places, the extremists having positions of power is relatively new phenomenon--even the modern Palestinian Resistance was mostly secular (around more socialist inspired groups like Fatah and PFLP) until the 1980's (Hamas was founded in 1987; Islamic Jihad in 1981).

tl;dr: there are plenty of Christian and Jewish extremists. However, in our popular imagination, we tend not to conflate Baruch Goldstein with our dentist Dr. Goldstein and we tend not associate Eric Rudolph with Catholics writ large.

While it's easy to find evidence of extremist groups belonging to other faiths, OP is fairly recognizing that Islamic extremism has a notable place in our world today. Other faiths have seen extremist wings commit serious acts of violence, though these don't preoccupy national governments on such a scale today.

Before I go into my explanation of why Islamic extremism in particular, I will say that many scholars of insurgency/ statebuilding/ terrorism generally divide the history of terrorism into four 'waves.' We don't generally count terrorist groups or violence similar to modern terrorism before the 19th century, as they cannot be unified under a single worldwide movement, and for that matter terrorism as a concept is relatively recent.

The first of these waves is the anarchist movement of the late 19th/ early 20th century. This movement was seeking to fight oppression, and uniquely did not begin a 'conventional' revolution but made their mark by single assassinations of leaders or policemen, culminating in infamous incidents like the death of President McKinley and Archduke Ferdinand. One could see then, individuals such as yourself asking why anarchism alone bred these types of attacks--my very short answer is that the movement had popular support across borders, and the use of such violence simply lended itself to educated, disaffected people.

The second wave accompanied national independence movements before, during and after the Second World War. Someone else will be better equipped to explain why decolonization worldwide took hold when it did, but we do know that fighters who had learned guerrilla techniques during the Second World War and its preceding conflicts applied their lessons to their fights for independence. Terrorism in the name of the creation of a Jewish state took hold at this point, and other examples include the Algerian war of independence, and terrorism in India.

The third wave blurs with the second a bit--it is defined as the wave encompassing Marxist movements. It includes Mao's insurgency, the Malayan emergency, Che Guevara, and the LTTE. Here we see that national independence movements were sometimes funded and supported by existing Communist countries like the USSR and PR China, fueling this wave globally. I say it blurs with the second because some independence movements (i.e. Zimbabwe) happened to tie Communism into their ideologies.

This brings us to the fourth wave, beginning roughly with Palestinian insurgencies in the 1970s and 80s, and evolving a great deal since then. My answer for this is a little complex, but I will attempt to mention every theory posited within it.

The first factor is globalization. The Middle East was particularly affected by the beginnings of the current wave of globalization, as it resulted first in economic and cultural changes, then unprecedented socioeconomic inequality. Surprisingly, however, the grievances that draw militants to commit terrorism are not usually associated with their relative poverty (to put it very simply, an insurgency marches on its stomach), but by newly educated and enriched people who find that their political power has yet to catch up with their new wealth.

In most countries, growing dissatisfaction was met with democratization or repression. Most governments in the Middle East chronically excluded a great portion of the population or actively suppressed them—and remained in power with the aid of the West (I mention this to better explain why the West becomes a target later).

Now this explains why an educated group might turn to political violence, but not many of the characteristics we see of the modern wave of terrorism. These groups coincide with those that see globalization and modernity as incomparable with fundamental Islamic values. Now the West becomes a double target as not only the supporter of repressive Arab regimes, but also an entity seeking to destroy culture (as an example, picture coming home in Cairo from work, feeling frustrated that day over your lack of political power, and walking into your house and seeing your son watching Baywatch). These groups advertised Islam as the only global brand that could counteract the ever-advancing wall of Western culture. The turn to Islam affects the discourse of the political struggle—if the struggle is transformed into one of a traditional religion versus secularism, not to mention secularism already associated with repression and exclusion, it is transformed into a cosmic battle of good versus evil for the militants. Leaders of emerging groups of dissent would happily adopt Islam as the vehicle by which they could gather support, coopting radical clerics and people who already believed Islam necessarily requires violence to defend itself (think Sayyid Qut'b).

So we have a radicalized and alienated group that seeks societal change, that has accumulated the wealth they need to actually stage a conflict.

Now what makes them terrorists? In Iran it didn't--the group was large enough that it could organize a majority of the population and stage a popular revolution. But in most other countries in the Middle East, these groups remained in the clear minority. So they would employ high-visibility attacks to invoke government repression, causing more people to feel that opposition to the government is necessary. The other side of it is intimidating the population into supporting them.

To summarize, the unique environment of the Middle East created large groups of educated but politically excluded people. These people turned to Islam as a vehicle for gathering support and radicalizing their followers.

Now, I approach this from a perspective as a comparativist, seeking to find answers as to the large scale movements across history. There is no shortage of exceptions to this rule, but I hope this illustrates how waves of terrorism come about and perpetuate. If anyone has issues with how I wrote this or anything I said, I'm happy to address it or try to find out where we diverge. There are a few things I also left out.

For citations, I have several (this was my thesis topic), so I'll hold off on those until someone needs something specific cited if that's alright.