Bruce Schneier gives three main reasons:
One, terrorist attacks are harder to pull off than popular imagination -- and the movies -- lead everyone to believe. Two, there are far fewer terrorists than the political rhetoric of the past eight years leads everyone to believe. And three, random minor terrorist attacks don't serve Islamic terrorists' interests right now.
... So, to sum up: If you're just a loner wannabe who wants to go out with a bang, terrorism is easy. You're more likely to get caught if you take a long time to plan or involve a bunch of people, but you might succeed. If you're a representative of al-Qaida trying to make a statement in the U.S., it's much harder. You just don't have the people, and you're probably going to slip up and get caught.