Because mass murder is ok if you do it to your own people.
Liberals don't oppose Nazis for being totalitarian mass murderers. They oppose Nazis for being racist. Murder doesn't figure as a crime in Leftist ideology, racism does.
Maoists, the Viet Cong, and the Khmer Rouge are acceptable to the American Left because their mass murders were not racially motivated. American liberals used to feel the same way about Stalin, but he's fallen out of the favor with the American Left because some of his acts of genocide were ethnically rather than just ideologically motivated.
Essentially, the Left forgives mass murder as long as it's driven by ideological class hatred rather than by "racism" (at least when that racism is practiced by whites against non-whites, when it's the reverse, Leftists are all in favor or racially motivated mass murder).