There are a variety of reasons, but in my worldview:
1. Yep, previous administrations are also to blame. There's been a thorough fuck up at every level for several decades. Happens when one government wants to plan the governments of several other countries for them. (See previous CIA missions putting dictators in power to get rid of current dictators... who then turn out to be al Quaeda.)
2. Bush is worse in my opinion because he's even stupider, and because he has taken these events as excuses to do what he wants to do. Do I think the previous Iraqi regime sucked? Yep. Do I think they had anything to do with 9-11? Nope. But that doesn't matter. He uses this, and uses all these scary things to TERRORIZE HIS OWN COUNTRY. People are much more sheep-like when they're constantly frightened. All of his "rhetoric" (quotes because he can't speak intelligently) uses classic brainwashing terminology, making it US and THEM and generalizing. And let's not forget how he's been stomping all over our rights as citizens.
3. Previous admins messed up. This admin messed up, lied about it, and then lied some more. Utterly disgusting.
4. Don't forget, most people say "Bush", meaning "Bush and all of his administration".
Just one person's view. There's more, but if I want you to read it...
