First of all, none of them were actually communist. They were probably actually closer to fascism. I think, however, fascism is a much greater insult for three very simple reasons. I) We've been to war with fascism. II) We associate fascism with Nazism. III) They're associated with monolithic, overbearing governments and fanatical racism. When you look at things like Stalin's regime you see something somewhat ramshackle, less threatening and simply not as fanatical.
All three of those can also be the same with IRL Communist regimes