The major reason for this is that fascism and communism have very different ideas about how people should relate to one another.
Fascism believes in competition between people. It believes in a type of Social Darwinism in which people compete and the strongest come out on top. This is in complete opposition to what happens in communism. In the communist system, there is supposed to be complete equality. No one is supposed to try to get above anyone else in society. To Hitler and other fascists, this sort of a society would become degraded because the lack of competition would cause people to become weak. For this reason, Hitler was strongly opposed to Bolshevism.