It's all because of the kind of bully he is, and what, in psychology circles, they call "learned helplessness".
This quote from a Jack Womack I saw over on William Gibson's blog brought it home:
We can see that the person now in office has led us into a terrible situation, and clearly has no idea what to do, and if reelected will continue to do more of the same. But his opponent has not given us a sufficiently exact plan of action indicating what he intends to do during the next four years -- regardless of what events might take place in the meantime. Therefore, obviously, the right thing to do is to stick to the idiot we know...With a bit of distance, it's completely obvious they are being held to different standards. It's like the entire nation just needs to be comforted and reassured that it really is okay to make a change. This election isn't so much about battling Bush as it is battling our own fears. We just need to succeed in shaking off the cobwebs, and then after that it will become a lot clearer.