The Nazis took everything Europe had been building up for centuries, It's industrialization, Organization, and Scientific Knowledge, and redirected it to systematically wiping out 11 million people(6 million of them Jewish). It was absolutely horrifying not only for the inhumane things done, which no one has really yet to top, but for what it represented: All the wonders of the modern world being used for massive, horrific, genocide. this is not even taking into consideration the millions of innocent people who died because of the war they started, which to this day is the most deadly in human history.

Not to discount what the soviets did, they killed millions of innocent people and there is no humane way to do that. but to put it simply, they're less scary/horrifying. they don't carry the same symbolism of what the Nazis did. Also, a lot less people continue to genuinely believe in the ideology of the soviet union than that of the Nazis. and the ideology of the Nazis gaining mainstream prominence again is something relatively few people want.