the western world, and specifically America, is the symbol of all that is evil and wrong in the world. If you're a fundamentalist extremist and think you can get away with a major attack on that symbol in order to reinforce your way of thinking and put yourself into a major position of power in your circle, why not?
this wasn't the first attack on a world trade center, or the first time al qaeda had attacked U.S. interests.
I remember reading something about Gary Condit engineering the entire attack to draw the media away from him during the Levy murder investigation. That one really cracked me up.