The thing is that there a lot of direct victims of Soviet crimes or their relatives who are still alive and remember their atrocities.
The Nazis on the other-hand generally just murdered everyone they hated (losing all of your property + 10 years in the gulag wasn't really an option if you were a Jew in eastern Europe...).
The Soviets were pretty bad. Allying with the Nazis against the Soviets would have been less wrong.