Who knows? Why did they wait so long between the two attacks on the WTC?
There is a reason. Answer that question and you might figure out the timing of the next attack.
You don't think they are done attacking us, do you?
I suspect they want to have a measure of our resolve. If they get the idea that we won't retaliate against Islamic holy cites or widen the war in a spectacular fashion, I suspect they will do it.