In addition to reasons already cited in comments, I think that we should also consider that Londoners never had the invincibility complex that most of America had. I think that a good part of the reason that people in America are all "OMG SPAZ!" about the whole thing is that we've been brainwashed for so long to believe that A) shit like this doesn't happen, and B) even if it *does* happen, it can't possibly happen to *us*.

London, on the other hand, has been rather intimately involved, for a very long time, with people that make Al-Qaeda look like amateurs.