The major problem is religion. Maybe not all religion, but religion has interfered with the the acceptance of Evolution since the idea was first introduced. This issue is not limited to America either. Huxley debated Wilberforce in 1860, right at the dawn of the modern study of evolution (http://en.wikipedia.org/wiki/1860_Oxford_evolution_debate). Even today there are groups in Europe that continue to deny it, though the do get a less welcoming approach in public than their counterparts in the US.

Some religious believers accept the theory, but no all. It took the Catholic Church a long time to come to terms with it. And even now there is a hard core of the Church that see the official Papal endorsement as a mistake. It takes a special kind of crazy to tell you that a guy is infallible who still makes mistakes, and fail to see the contradiction in that stance. (And yes, I know....the Pope didn't claim infallibility on that one. I'm just saying.)

One difference between he US and Europe though is America has played at the "everybody is equal" game for a little while longer. I think we sometimes commonly make the mistake in assuming that "equal before the law" means the same as "everybody's equal". And that leads a person to assume that their uninformed but deeply felt belief is equal to an evolutionary biologists expert opinion. And therefore some assume that means they cancel each other out and then they can choose to believe the one they like better.

So, yeah.... I'm cycling back toward religion makes people believe some crazy things.