Good point, Elisabeth.
I also think, however, that a lot of this has to do with timing and secrecy. The holocaust was no big secret after the Americans and Russians got to the camps, accompanied by photographers and reporters who quickly spread photos and news stories across the world.
In addition, the entire Western world was involved in WWII. Each victory or loss meant something personal to the citizens of the US, Britain, France, etc. Because of our soldiers, who were spreading out across the continents, we had a sense of connection to whatever was uncovered/discovered/determined.
Mao and Stalin, however, did not welcome journalists, enforced censorship, and essentially did not allow the visual and first-hand stories to escape their control. The vast majority of people in the world did not connect or relate to the peoples of Asia and the tremendous paranoia about Communism in the US, at least, made people avoid the subject of Russia altogether for quite some time.