Ton of reasons, some off the top of my head:
(1) Bush had higher approvals, but everyone could see it was all on a wartime bounce. Obama is holding steady at 45-50% even during the lows of 2010.
(2) Bush won a contested 2000 so there was still a strong "wronged Democrat" opposition
(3) Though Dubya had a powerful and united conservative machine run by Rove, he was no way near as skilled of campaigner as Obama. Kerry lost by 2.5% on Election Night, but was down by 10 points before beating up on Bush in the debates. I doubt Mitch Daniels could beat Obama in a debate.
(4) The demand for ideological purity in the GOP primary this season thanks to the Tea Party is making it incredibly difficult to get nominated and still have an ounce of dignity for the general election. Primary ideology shifts are normal, but there's no place for individualism this cycle.
For example, Bush was pro-amnesty in the immigration debate back in 2000 and 2004. He wouldn't be allowed to get away with that in 2012. There are a lot of votes and signings these Senators and Governors have on their resume that don't mix well with tea.
Obama wins and rides out the last four years of the New Deal era.
The New Deal era died in the early 80s.
