A good question, Sherman.
Sheer ignorance has to be the major factor, and sadly, that's often ignorance that is rooted in religious belief.
As demonstrated above, some people simply refuse to actually get even a basic understanding of what evolution
actually says preferring to believe lies and misrepresentations because they apparently fear the consequences on their religious faith of accepting evolution. This is, you will note, despite the fact that there are plenty of theistic evolutionists who seem to have no problem with the concept.
The fact is, until people actually understand the actual theory rather than strawmen, it's going to be difficult to get people to change their minds. It's one of the reasons I'm such an annoying git about correcting lies about evolution on this site.
As for stats, here you go:
http://scienceblogs.com/pharyngula/upload/2006 /08/public_acceptance_of_evolut.gif