Because they lost, and it is free to criticize them. Also there were survivors.

Stalin killed millions of their own people too and how many films you see about it? Few films about China revolution too. It was not politically correct.

Part of my family comes from a communist country. If the party came to acknowledge that you stored pictures or any other document or proof of "past errors" you will have lots of problems, so most people destroyed them. Delation was normal even between family members.

When you kill them all, there is very few people that could write the History. Most of what we know of the native Americans genocide comes from the very few Americans that opposed that. But most Americans were certainly ok with killing the people there if they and their families could profit from it.