The reason is that roots of communism and nazism are different. Marx wrote Communist Manifesto in 1848 and it influenced working class movement in many ways, which, one can argue, has led to several good things in society. It also influenced the horrors you mentioned. Nazism, on the otherhand, is largely associated with Hitler, although it has roots in pan-Germanism and nationalism in general.

Thus, communism is associated with a wider spectrum of ideas and events than nazism, and because of that expressing some forms of communist ideas is tolerated in the modern society better than expressing the Nazi ideology.