Tokensim and cowardice.
Tokenism - lets people posture, morally, by selectively focusing on what the Nazis did.
Cowardice - takes no courage to highlight the Holocaust, because the Nazis were defeated, while the contemporaries and heirs of Stalin were not, their crimes were covered up (because they were "allies" of the West) - it would have taken actual courage to broadcast the truth about Stalin's crimes, and so we get Putin, among other things.