The biggest obstacles to people accepting evolution as a fact are:
1. Ignorance. They don't know of the mountains of evidence that exists and proves it. They actually believe that it's "just a theory".
2. Misinformation. They've been misled by creationist propaganda that consists of fallacies, lies and outdated information.
3. Lack of understanding. They don't really understand evolution, although they think they do. The evolution that they find hard to believe truly is a myth because it's not really evolution but their false understanding of it created by their ignorance and by being misinformed.
It's always easy to tell when an evolution-doubter is ignorant, has been misinformed or lacks understanding. They ask rhetorical questions that someone who is truly knowledgeable of the subject of evolution would not only not ask, but would be ashamed to ask - unless as a joke.