Regime continuity. When Hitler died, his regime soon followed. Immediate evidence of the regime's heinous deeds was available, both on film and in the presence of the regime's enemies. The following German leaders exposed the crimes, both DDR and GDR. Both demonized the other using the Nazi regime as a malignant example.

However regime continuity dulled evidence of the other regime's crimes. The leaders that followed Stalin and Mao needed continuity to provide legitimacy; they could ill afford to denigrate Stalin or Mao. Even when Khrushchev denounced Stalin, he did so secretly; most people of the USSR remained ignorant.

Propaganda played a part. To the current day, you can go to Dachau and see the ovens; starving Chinese and Ukrainians leave no museums to infamy behind.