My specialty is in terrorism during the specified time periods, but I have read a few more modern works which have suggestions about this very issue. I can tackle your three examples one by one, and then talk about the issue in a wider context.

Let's get some super important but boring stuff out of the way. A lot of the nuance to this question is going to come down to terminology and how you want to use it. For example: just the phrase "domestic terrorism" is going to be sticky. What do we qualify as terrorism? When does an internal conflict become a civil war, or a revolution? Were the Contras terrorists? The IRA? Where do we draw the line between different types of combatants, and what's our criteria? "Domestic" is going to be challenging as well, especially as we progress into the contemporary era with more non-state actors perpetrating these acts. And don't get me started on the conflict in Israel. Are Hamas rocket attacks on Israel "domestic"? Are they terrorists, or combatants?

So I'll show my hand and say I want to tackle this question by using examples of what I'll call "militant movements" whose actions are often regarded as terrorism. This is not an attempt to legitimize them, but to make the difficulty of the question clear. the vast majority of my examples will be from the Western world as well, given that's my area of research.

But first, let's start with hijackings, and address the elephant in the room: 9/11. The role of hijackings, usually, during the Long Sixties was more negotiation or extortion. You take a plane, you demand some of your friends get out of jail. Now, you certainly had ideological groups who targeted particular types of people, no doubt: Palestinian organizations like the armed wing of the PLO occasionally let go non-Jewish passengers. Still, they had a demand in mind and more often than you might expect given our current "don't negotiate with terrorists" mindset, they got some part of what they wanted. Now, it might surprise some people to know that by 1999, the world saw 13 hijackings, down from a few dozen a year during its peak in the late 1960s. This drop has to do with better screening techniques, especially in hot zones like Europe and Israel. Security became much tighter on international flights in these areas. But do you know what didn't change? The treatment of domestic flights, especially in the United States.

9/11 changes the hijacking game. We get to see just how dangerous allowing determined individuals to take control of aircraft can be. Now, both the attempt and goal of such an action is fundamentally different. Even if a determined individual makes it onto their target plane, pilots are trained to not allow them access to the cockpit. That door isn't opening easy, force be damned. And even if you do get access to the cockpit, don't expect to get your demands listened to. You're not gonna be allowed to take that jet to Cuba, especially if you might pass over large urban centers. The National Air Guard trains for exactly these types of scenarios, and they're trained to shoot if necessary. This is not to say attacks on airplanes had never been of murderous intent: the Lockerbie bombing comes to mind. But I'm sure you can see the difference there.

As for bombings: let's start with the state of militant action in the West during your time period. High profile bombings by organized left wing groups like the RAF, IRA, ETA, Weatherman could be weekly occurrences; in some places, daily ones. However, it pays to remember that - if we exclude the IRA and to a lesser extent ETA - these groups committed actions which don't constitute huge losses of life during any individual event. Weatherman stops targeting human life after their members were killed by a premature detonation; they would call-in about their devices. The RAF lasted almost thirty years and set off about 300 bombs, killing (according to Jeremy Varon's "Bringing the War Home") 34 people. They also had complicity in the Munich Massacre. Now, don't get me wrong, as all killings are tragedies. But the social and cultural impact these organizations have has a lot to do with the political climate they found themselves in. They were high profile for reasons beyond planting explosives. Many of those New Left militant movements were made up of middle class college kids, which plays into the deep fascination many have about them.

Still, hundreds of bombs in Western Germany, thousands more in Northern Ireland and Spain... those are staggering numbers, no question. Why don't we see as many bombings anymore? This is actually a question Im currently writing on, and I want to suggest three reasons: the problem of organized resistance, the security situation, and our classification of terrorism.

What types of issues lead to militant action? Let’s look at three general categories: ideological motives, nationalist/separatist sentiments, or to force a change in a particular policy. Of course, ideology is going to play into the other two categories as well, but what matters most is the end goal, and that end goal often manifests in the actions perpetrated by the militant. For example: let’s take Group A whose stated goal is “demolishing the imperialist and capitalist system that exploits the third world”. Sounds lofty, right? It also sounds monstrously difficult, and one that lacks a clear endgame. Group B, on the other hand, wants to reunify their occupied state with the country they consider rightfully theirs. Okay: now we have something more concrete to go on, and might be able to craft a more coherent battleplan. And finally Group C, who wants to harass the government into allowing its members proper civil rights. Maybe you’re filling in the blanks with the familiar suspects… but surely these problems still exist? Why were organized groups fighting for them then – with differing levels of efficacy – but not so much now?

This is where the other two reasons play their part. First off, I want to resist the notion that “bombings” in the abstract are ludicrously less common. Of course we have the high profile domestic attacks in Europe and the United States following the end of the high-period of domestic terrorism: Oklahoma City, the Atlanta Olympics, UNABOMB, etc. However, there are still a significant number of attacks perpetrated by domestic actors on a regular basis, especially in the United States. Pipe bombs and grenades placed into abortion centers; bombs and arson attacks committed by eco-activists/terrorists. These types of things aren’t one off events. There have been over 40 bombing attempts on abortion centers since the 1980s with around a dozen deaths, and those don’t take into account other forms of militant extremism like physical assault. A single group of eco-militants, the Animal Liberation Front, have committed over 600 acts to the tune of $43,000,000 dollars, according to the FBI archival files. And that’s an animal activist group: other organizations target development projects and logging sites. Estimates for their damages range into the hundreds of millions since the early 2000s.

All of which is to say militant bombings are not uncommon occurrences. Absolutely, the heyday of organized domestic action is in the past. But what caused this change? And why are we not as aware of these actions, when – statistically speaking – some of these Eco-groups have performed more successful bombings/arsons in a decade than Weatherman did in two? Well, the profile of course is very different. Burning down a half constructed suburb in pristine Colorado might not make the NYTimes like a bomb in the Capitol Buildings bathroom. Moreover, the pathology of these actions tend to be different. While these leftist groups were organized and often had a central command structure, the improvement of counter-terrorism tactics forced groups to evolve. Cell structure and “leaderless resistance” are more common now, and you can see how this relates back to the importance of motive and intention. Timothy McVeigh is written off as a “lone wolf”, and his one – hugely devastating – action an independent problem. But this isn’t true: the anti-government ideology McVeigh followed was not devised by him alone. While there might not be a prolific “AGA (Anti-Government Action” group with a command structure and unified right-wing ideology, any independent acts can be traced back to similar ways of thinking and planning. It’s more that the optics have changed due to the changes in counter-terrorism. What about religious ideology? Plenty of domestic attacks in Europe are carried out by radicals claiming allegiance to ISIS, even though they were born in the country they carry out their actions in. While some receive material support from those groups, others merely attribute their motives to a non-state actor. Moreover, given that the intention of these attacks is often the indiscriminate loss of life, or are perpetrated with the expectation of capture/death, they look very different from Andreas Baader burning down a department store.

There is a great book, The Skies Belong To Us , on hijackings specifically that examines why there were so many between 1968 and 1972. One of the fascinating tid bits from the book is that hijackers headed to Havana were so common that there was a direct line between Miami and Havana air traffic control to deal with it. It is written by a journalist, not an academic, but is well researched.

The book gives a number of contributing factors to the prevalence of hijackings, that fall into 2 main buckets.

Cost and Convenience The powerful airline industry lobbied against security measures such as X-ray machines and metal detectors because it would increase costs and turn people away from flying. Violence during hijackings was rare, and so many authorities believed the best policy was simply to cooperate: Some domestic flights were stocked with maps of the Havana airport and Spanish phrase books in anticipation of hijackings. On average a hijacking cost the airline just $25,000, and so it was simply priced in to the cost of doing business.

The People In The Era There were a lot of people disaffected by the Vietnam War in sometimes desperate circumstances that saw hijackings as a way to make a point and/or make some money and get to somewhere new. The book makes the point though, that while many hijackers professed to have the sort of hardcore or radical beliefs of the era, those beliefs were often just held temporarily. They were also encouraged in part by at least part of the nation that sometimes saw them not as criminals, but as striking back. Hijackings were so regular that it was just a regular part of life and a subject of late night jokes, and its ubiquity inspired copycats.