Why does the modern radical left not display the fierce hostility to Islam that the 19th century radical left displayed to Catholicism?

So unfortunately I think a proper response to your question would require a knowledge about:

Islam (which I know a thing or two about)
20th century left wing movements (which I have a well-informed layman's opinion about)
19th century left wing movements (which I know very little about)

and 4. 19th century left-wing anti-Catholic anti-clericalism (which I know nothing about.)

So to say the least it would require a broader range of knowledge than I possess to answer the question fully.

The being said I think I can discuss a few issues that point in the right direction:

Among 20th century left-wing movements (and I'm avoiding the use of "radical" left because then presumably we're talking a violent subset which would require a level of knowledge about, say, the Red Army Faction or the Weather Underground that I simply don't have) one of the defining features of these movements has been anti-colonialism and anti-imperialism.

The overwhelming majority of the Islamic world for much of the 20th century was part of the European imperial system, either directly in the case of Greater India, as a kind of suzerainty in the case of Egypt after it was nominally granted independence in 1922 or as a playing field for Western imperial powers to do as they please (e.g. the occupation of Iran in 1941 and the coup there in 1953.)

So when you have anti-colonialist movements in the Islamic world, many of which are in fact not overtly Islamic or Islamist, these were naturally embraced by the anti-imperialist Western left. The 1956 Suez Crisis I think is a great example of this where Nasser, a secularist leader of an Islamic country was seen by the left as resisting a Western imperialist plot against him. As it turned out, that analysis was exactly correct. But the result was anti-war protests in Britain looked a lot like the kinds of left-wing anti-war protests that I think the popular imagine are more commonly associated with the 1960s.

I think shifting perceptions of the left vis a vis Israel are also crucial here. Zionism was, for a time, very much viewed as a left wing cause. The Zionist movement in Palestine prior to and after the declaration of the state of Israel was largely dominated by Labor Zionists who in the form of kibbutzim were promoting the kind of collectivized farming and communal living that was then current with international left-wing movements supportive of various forms of socialism or communism, for example in Castro's Cuba.

Israel, in the aftermath of the Holocaust was likewise seen as perhaps the greatest example of the national self-determination the world's most oppressed people, perhaps the most oppressed people in world history. So just as the US recognized Israel minutes after it declared independence, the Soviet Union was also incredibly supportive of Zionism and I believe was the first country to de jure recognize Israel a day or two later.

In varying degrees between 1948 and 1973 that left-wing support for Israel started to change pretty significantly. Firstly, the Middle East itself became just as much a theater of action for the Cold War as any other part of the world and which became closely related to the so-called Arab Cold War. So that, in the build-up to the Suez Crisis with Nasser recognizing the Chinese communist government, receiving an arms deal via Czechoslovakia, it looked as though Egypt and the Arab nationalist states would be increasingly the preferred bloc of the Soviet Union and the international left while Israel and the Arab monarchies would receive the support of the US and Europe. (NB, this is oversimplifying things, and in particular the US relationship with Israel between 1948 and 1967 is actually really complicated, but in broad strokes this is how it turned out.)

After 1967, Israel also starts to look to the left less like the state of an oppressed people and more like a Western-backed oppressor in its own right. Also note that this is less true of the US left, where even if support for Israel ebbs and flows, support for Arabs was virtually non-existent until the late 1970s.

Following the 1967 there are a number of developments that more closely align the Palestinian movement with the left. The Popular Front for the Liberation of Palestine is founded in 1967 and Yasser Arafat takes over as chairman of the PLO in 1969.

Likewise in this time period you also have, specific to France, the Algerian revolution, which to my understanding very much became a left-right issue.

By the time of the 1973 war I think any doubts about the extent to which Israel was going to permanently receive the support of the US were over, and as a result the anti-American left also became a pro-Arab/Palestinian left. This I think you could make the case was reinforced by the premiership of Menachem Begin in 1977, Israel's first Likud (i.e. right wing, non-labor) prime minister.

On the other side of the ledger, by the 1970s you also have the makings of a religious revival in the Islamic world. Secularist Arab nationalists are starting to lose ground to Islamist factions of the Muslim Brotherhood so that even where pan-Arab socialism is still dominant you have leaders like Anwar Sadat making significant concessions to Islamists.

I'm not familiar with what left-wing support for the 1979 Islamic revolution in Iran was like but that revolution certainly borrowed much of the language and propaganda elements of the hard left including national liberation and anti-Americanism. From about 1979 onwards international politics in the Islamic world I think starts to look a lot more Islamic, so that while leftist groups dominated the Palestinian movement in the 60s and 70s in the 1980s you have the emergence of the "Axis of Resistance" between Hamas and Hezbollah.

So I think across the region we can see that support for movements that had once been left-wing and secularist are now increasingly Islamist, but that international left-wing support for those movements remains in place.

I think that's the portion I'm most comfortable discussing, which is to say the evolution of anti-imperialist and anti-colonialist (whether actual or self-described) movements in the Middle East and some of the international reaction to them.

I would also point to a number of topics about which I have much less expertise and would love to hear someone else chime in:

The role of cultural hegemony in left-wing views of oppression. Whereas the Catholic church was an institution of power in the west that was opposed as such by the left, Islam has never occupied a position of power.
The plight of Muslims in the West. Likewise, where except in narrow circumstances Catholics were not viewed by the left as an oppressed minority, the same cannot be said of Muslims in the west, and the left has often tried to stand up against Islamophobia or racist attitudes and in support of specific Muslim communities (e.g. French Algerians, British Pakistanis, etc.).
Concepts of intersectionality in left wing discourse. Even before the term itself was coined there were clearly political issues of an intersectional nature on the left that related to the left's views of Islam and issues in the Islamic world, for example, the relationship between the US Civil Rights movement and Black Power movement with the Nation of Islam. Or Israel's relationship with the apartheid government of South Africa prompting pro-Palestinian solidarity among anti-Apartheid activists who might otherwise not have been involved in the Palestinian issue. Since then broad swathes of the left have explicitly embraced concepts of intersectionality, but that's not a history I'm entirely familiar with.