That's because the communists see fascists as a primary competitor. Fascism stems from the same exact root ideology as communism, and the overlap between the methods of government is huge. But much like how a totalitarian believes themselves to be the de facto ruler that can do no wrong, so to do fascist and communist nations. Thus, barring external threat, those nations will ruthlessly attack each other in order to take power from them in order to further their own personal agendas.