Two things happened after 2001 to disrupt the terrorist strategy. First, there was the 2003 invasion of Iraq, which forced Islamic terrorists to fight on their own ground. This led to Moslems fighting Moslems, as other Moslems had always been the most numerous victims of the terrorist attacks. The main reason for turning their attention to the West was the inability to make much progress against the rulers of Moslem states. The Islamic terrorists considered these leaders corrupt and tyrannical. They were correct in that assessment, but these despots were also very good at suppressing any dissent, and Islamic radical organizations had suffered several major defeats in the 1990s (mainly Algeria and Egypt). The West was considered more vulnerable, and operations in the 1990s seemed to confirm this.
But Western counter-terror specialists had an ally that had developed an effective strategy for disrupting Islamic terror attacks. Israel, under attack by Palestinian terrorists since 2000, had found a vulnerability. Islamic terrorists who know how to build bombs are relatively rare, as are several other kinds of specialists. Far more people are willing to plant the bomb, or set them off, tasks which require little skill. The Israelis quickly figured out that, if you want to stop terrorist bombers, concentrate on the bomb builders and other skilled people. The Palestinians didn’t have too many of these specialists to begin with, and they were often highly educated, or simply bright, individuals who applied their skills to the dangerous task of building bombs and getting them to targets.