I'll take that one, Moose.
Peer pressure. Really. The accepted perception of Bush is that he isn't too bright. So, in order to appear "with it" or "above it all", people appear to have to have a detachment from him. Some have to be so above it all they have to detach themselves from both parties at every opportunity. It's cooler to be "independent".
I didn't particularly like Bush as a first choice, but I am pleased pretty much so far in his presidency, and if the Dems can't give me a better choice I will vote for him this time. (You see, last time I voted against Gore).