It's not fashionable to say so, but -isms of the left (Stalin, Mao) killed far more people in the 20th century than the -ism of the right (Hitler) did.

Why do we not excoriate Stalin's and Mao's latter-day followers (communists, socialists) and label them with their fathers' genocides as we do with Hitler's?

Because we're unthinkingly fashion-led, that's why.