I think a lot of it has to do with the fact that we were at war with Nazi Germany and soundly defeated them, and held up all their atrocities for the world to see as we tried and hung the surviving perpetrators at the end of WW2. Communism on the other hand, flourished in the shadows. Sure, there were some expose’s written about the gulags, but they never were defeated in as open a way as the Nazis were. Also, because we were allied with commies in WW2, our own propaganda portrayed them as somehow more benign than the Nazis.
There are so many WWII movies I lose track of them, but there was one recently which revolved around the Soviet massacre of Ukrainian officers. The theme is how the Allies helped hide that atrocity since Russian was their ally. So maybe things are changing?