That question is too often asked rhetorically. The answer that is implied is that there is a “clash of civilizations” and that Islam has been plaguing the West with terrorism ever since its birth in the seventh century. The question usually hides an anti-Islamic bigotry born of ignorance of both Islam itself (and that includes the very nature of religion) and history.
I have answered the question “why terrorists are generally Islamic” in The Origins of Islamic Militancy — for those seriously interested enough to truly understand.
Of course the fact is that throughout the 60s, 70s and 80s the only terrorists most of us ever heard about were fighting for national liberation and Islam per se rarely if ever entered the picture. Islamic terrorism is unquestionably a new phenomenon that has burst on the scene quite recently in historical terms.
I have added to my original post, The Origins of Islamic Militancy, since its first posting. I have added a section on global jihad and the author of this concept, Abdullah Azzam. I have one more section to add to this post and that will be Osama bin Laden’s contribution and what it was, historically, that led to his particular slant on what jihad should be all about. I will notify interested readers when I complete that section.