As soon as Bush's approval numbers soared after 9/11, the left began an incessant lie campaign to bring them down. They beat him up mercilessly in every way possible to make him lose the popularity that he had earned through his leadership.
It worked.
On the other hand, they have done everything in their power to buoy up Obama by never, EVER reporting all his scandals and nastiness, and yet his numbers have still gone down as low as Bush's.
It really is quite remarkable, isn't it?