Why has international Palestinian terrorist attacks ceased?
This has several reasons; they're not distinct from each other; and I will not list them in order of importance necessarily. Nor will I list them all.
First note that Palestinian terrorism is not in essence, and often not in form, "Islamist". It's rooted in a struggle against occupation and colonization. So even if part of the resistance to those occupation and colonization is by Islamic movements or forces, and even if there are religious overtones to some of the terror attacks by Palestinians, it's not actually about religion. This fact also explains why changes in the situation of the Palestinians politically/nationally changed the nature, location and frequency of the use of terror (regardless of your definition of what it constitutes), despite the fact that religiously, not much of significance has happened.
But to be more concrete, an important reason for the drop in terror attacks by Palestinians is that Palestinian political movements - at least their military wings but not just those - have been driven out or have left, partially or wholly, centers of activity outside of Palestine; most prominently, Jordan in 1970 and Lebanon in 1982 etc. They have moved or shifted their focus of activity into the 1967-occupied parts of Palestine. This is true first and foremost for the Fatah movement, and the PLO as an organization. It reflects on what kinds of activities they initiate and where. Before, they could mostly confront the occupiers on the (effective) borders of the Israeli state, or abroad. And abroad they could either stage feeble protests (I'm exaggerating here) or use armed actions - plane kidnappings, revenge assassinations etc. In Palestine itself the spectrum of activities, both unarmed and unarmed, is much wider, arguably more effective, and obviously more captivating of focus than trying to terrorize Israeli officials abroad.
There is also the Imeprialist-orchestrated proces of pacification and political dialog - the "Peace Process" as many refer to it. When you parley, you don't strike; or at least, not as much. Of course, not all political movements have been involved in this process - with the prominent example being Hamas, currently the ruling party (*). It was actually responsible for the vast majority of armed attacks against Israelis during the 1990s, the years of the Oslo accords and their gradual implementation until they either broke down or reached their necessary dead end. After that, parts of Fatah resumed armed activity as well (and this includes both actions against soldiers and against civilians). During the early 2000s, you had no parley and lots of attacks. But all these and subsequent changes are mostly within Palestine, not in the west.
A final reason is that we seem to have had be a sort of zeitgeist change from the 1960s and 1970s into the 1980s and later. Somehow people were under the impression that radical change was just around the corner, or potentially just around the corner, and some decisive actions could bring it about relatively quickly if not easily. That also seems to have changed in 1980s - although it had been rekindled in the Arab East in 2010-2011. But - not in Palestine, or not for Palestinians.
(*) - They won the last elections; the fact that a US- and Israel-backed coup attempted worked for them in the west bank doesn't change that. I don't support them one bit, by the way.
And is international islamist terrorism now exclusively Sunni?
As others have noted, there are not-so-many Shia but lots of Sunnis. There's an even lower Shia-to-Sunni ratio, I believe, among people visiting or living in the West. So it would be quite anomalous for there to be many Shia attackers.
Also, the US is quite busy waging war in multiple Sunni-majority countries, and it's been that way for, what, over 15 years straight. Now, you do have Iraq, but after occupation a Shia or Shia-oriented government was installed, and IIRC the US remained militarily active more against Sunni(ish) elements there. The US has not attacked Iran, and has not managed / not tried to organize any sort of coup since the 1950s, so despite Iranians likely not being very fond of the US (justly or less so), I would not expect any of them to look forward to attacking it. Even more so with Europe with which Iran has nearly ok-ish relations.
PS - Note that some attacks, particularly some attacks attributed to Shia (or to Shia entities like Iran) are not necessarily proven to have been carried out by them. For example, the 1994 AIMA bombing in Argentina has a rather bizarre history of intrigue surrounding its investigation; the supposed motivation for the bombing for Iran doesn't square very well with preceding events; and no trial has been held nor any person convicted (although I think that's probably due to the implicated people not being around).