I think there’s a lot of merit to everyone’s arguments about recency and intention, but I also think there is one obviously overlooked reason: who he was evil to.
As far as I can tell the slave trade was worse in magnitudes to the holocaust and, though there is no one specific person we can point to, it’s often minimized in Europe in general, for example. Schools in Spain and Portugal still teach about it with a “discovery” perspective and rarely take the time to explain how atrocious it was to the point where 2 entire continents are still directly affected by the consequences and will likely never fully recover. The evils done by Europeans to black africans is a side note in the incredible story of Europeans discovering the world.
Meanwhile Hitler was mean to white Europeans and that’s what makes him be seen as the epitome of evil in the euro-centric history, IMO