Of course, when I ask myself why, I can only answer based on what I know, and, not having been involved, I don't know much. But I suspect that this attack was not motivated by a principled objection to anything about America or the West--I do believe it was revenge for a specific event, though not knowing who was ultimately behind the attack, I can't say what event that it might have been--

This morning, I read something about the war in Afghanistan that I did not know, and am not sure if I believe--but it sure has given me a bit to think about--Specifically that the Mujahadeen, Bin Laden, and all, did not know that they were being trained, indoctrinated, and sent off to die by the CIA,hiding behind a veil that included Zia, and a number of fundamentalist Mullahs--The war against the Russians destroyed what there was of Afghanistan, cast the many tribes into exile, and left a legacy of many, killed and maimed. If it were your country, and you discovered that the "Jihad" was actually a cynical front for American action against it's long time adversary, I would reckon you'd be a bit peeved--

Just another possibility....