I honestly think Occam's Razor can be applied here.

The 9/11 attacks took a lot of time and resources, and a good bit of luck, to pull off successfully. It led to the US attacking Afghanistan and Iraq, which seems to have been OBL's strategy.

It's a heck of a lot easier to fight the US in the Middle East/Central Asia if one looks at the situation from a AQ perspective. You've successfully drawn the US into two wars, both of which are nowhere near ending, and have bloodied the nose of the US at the same time. While AQ has been mangled quite badly, they are still around, and have grown new offshoot organizations simply by not being eradicated. You can draw more manpower, more weaponry, and more resources from the Middle East to fight in the Middle East - it's simply easier to sustain and maintain.

They really don't need to hit the US again - we're pretty much engaged full throttle with our military, and our spending is going through the roof. The wars have costs $758B since 01 according to the GAO, and that would equal the GDP of the 16th largest country in the world as of 2006. The AQ strategy has always been to weaken the US economy, and while there haven't been any major economic disasters since 9/11, the economy certainly isn't as good as it could be. Plus we still have a massive national debt that is going to have to be serviced at some point in the future...