Because the American public is more willing to forgive presidents that don't project themselves as smarter then them.  Its similar to how Oliver North and the Irancontra scandal in the 80's did not affect Reagan's popularity, but how Clinton's "lies" about Monica almost resulted in impeachment.  In a way, this may be Bush's genius, that he understands what lies Americans are willing to accept, and not accept.
Why can Bush get away with misinforming us about intelligence, then use that as reason for war?