Because although the theory of evolution is heavily supported by evidence, it is still just a theory and not a fact. Don't get me wrong, I'm agnostic, but since Evolution hasn't been completely proved, creationists are still able to cling to their beliefs. In the case of the heliocentric solar system, there is now undeniable proof that the we revolve around the sun. If there was some doubt in the system, then creationists would completely reject it. I'm not really sure, but I think there are still people who reject the idea of a heliocentric solar system.