Here is a link to a recent study that says "Sizeable Minorities [in the US] Still Believe Saddam Hussein Had Strong Links to Al Qaeda, Helped Plan 9/11 and Had Weapons of Mass Destruction":
What we can see here is that people are too quick to believe what the authorities tell them, particularly those with whom they identify (in this case, Bush voters), and the second problem is that once people started to believe something it is very hard to "unlearn" an idea they have become accustomed to and have become fond of. This is particularly true for "simple explanations". Nobody wants to hear the complex truth. Simple and black/white is much easier.