Because Hitler vocally espoused a rival, socialist idealogy, railed against communists due to that rivalry, eventually betrayed and attacked the Soviets as a result, and the intellectual heirs of those communists have captured academia and the news media in most of the western world following WWII, writing the subsequent history and shaping popular opinion ever since.
That, and some deem class or religious belief mutable and therefore more escapable than race, as far as just who gets killed off. That somebody, in the millions or billions, gets killed off, is a given with such people. They're merely quibbling over the selection criteria.
Ugly, ain't it?
Quite. I agree 100%.
BTW, a good many people killed by the Soviets were murdered for ethnic reasons. Volga Germans, Crimean Tartars, etc. Even the Polish officers killed at Katyn were probably murdered as much for being Poles as for being officers.
The Nazis killed roughly 12M innocents. The Commies killed roughly 100M. Yet somehow the Nazis are considered the ne plus ultra of evil while the commies largely get a pass.