Just a couple days ago I was responding to an unrelated piece and found myself thinking about how so many Americans would roll their eyes/feign offense at the Confederacy being compared to Nazis, which lead to wondering what criteria we'd use to decide which atrocity was "worse"--slavery lasted far long and affected far more people, but at least there's a vile logic for its existence: it benefitted the masters. A large part of the Holocaust's horror isn't simply its scale, but its sheer pointlessness; it benefit absolutely no one, and if anything was a massive waste of time and resources for the Nazis at a time when their very existence was at stake.
Then I realized what I was doing and chastised myself for trying to Power Rank the worst tragedies in human history. It's not a competition. Neither should have happened. Full stop.
While White Supremacy was definitely a huge factor--most likely the largest one--it's a huge oversimplification to stop right there. There are a lot of other, more straightforward explanations for this:
1) The Holocaust was much more recent. Every argument about how recent slavery was applies even moreso to the Holocaust.
2) Thanks to changes in technology, the Holocaust was better documented. It's been thoroughly established that humans react far more strongly to audio/visual evidence than a written description. If we had video of actual slavery in progress, it would completely upend the way we talk about it.
3) We forced the Germans to confront their sins, but we let the Confederacy off with a slap on the wrist. We had an occupying force in Germany keeping a close eye on them to make sure the Nazis wouldn't reform or develop a guerilla ressitence. We re-educated them and threatened anyone remotely supportive of the Third Reich. None of that happened with the South. Because of that, they simply developed loopholes and workarounds to keep variations on slavery around the first chance they got.
4) It's easier for Americans to imagine atrocities in foreign lands, even predominantly White ones, that it is to imagine it on their own soil.
5) The Nazis just had the "Evil" aesthetic down cold. They weren't just evil in their actions; they dressed evil (they put a skull and crossbones on their hats, for God's sake!), their leader gave evil speeches in the voice of an obviously evil man, and so on. They were walking cliches, which lead to them becoming a semantic go-to when it comes to a metonym for Evil.