In the case of places like the USSR and Korea some of the lack of understanding of what happened is due to how effectively the signal was stopped. It took Solzhenitsyn many years in hiding to finish the
Gulag Archipelago and then most of a lifetime of exile. Most voices like his were never heard, just in an unmarked grave somewhere. In contrast, the whole world could not help but see the wake that the Nazis left. Everywhere destruction, many survivors free to tell their stories, the oppressors all dead, rounded up or fled to far corners. Stalin was way more efficient in suppressing the truth. Maybe Hitler didn't care and he actually believed his own hype that it would be just his cronies left standing to write the history.
Another factor that can apply is that the world just didn't want to see. Certainly a fact for places like Cambodia, sticking our collective heads in the sand was way easier than doing something. IN a world weary and desensitized to war we just let internal politics be internal politics, even if a million or so die. But yay for us, we weren't the world's policemen.
Same thing for Soviet Russia. How many readers did
We The Living get? The world preferred not to know. And the same would have happened to the Nazis if they had been content to overrun Central Europe and just kill Jews. They'd probably still be there and we'd still be ignoring their atrocities and talking to them, as is borne out by the West's willingness to engage and appease Hitler throughout the 30's. The nazis just reached too far and by doing so exposed their mess to the world.
