because Bush says so. Isn't that reason enough? Oh, and because we're imperialistic pigs, war-mongers, oil-greedy, power-hungry, baby-killing, strong-arming roughians who impose our "freedoms" and ethics upon the world whether they like it or not. Oh, and Bush wants to be re-elected.
Oh yeah, there was something about disarming Saddam as well, but that's just an aside.