I asked the question. A reader suggests the following:
Haven't you listened to Bin Laden? He wants us out of the Middle East and believes that a failed war there will do the trick, just like it did with the Soviets in Afghanistan.
Why would he provoke us with another attack when we're doing exactly what he wants? Namely exhausting our financial and combat strength fighting a war that we're not going to win, mostly against forces that aren't his and alienating ourselves from the people that he is trying to recruit. Do you think he wants to provoke us so much that we'll deploy enough troops to win?