If it's al Qaeda, why Spain? I know they're a big supporter of the WOT, but so is Italy and Britain.
My guess is that Spain is a little more "backwards" technologically than Italy and Britain; thus, it's easier to get bombs in place without being detected.Plus, we know that the Muslims still have a big grudge against Spain for evicting them some 500 years ago. If there's one thing that they're good at, it's holding a grudge for centuries and centuries.