Robin Hanson thinks it the result of a signaling spiral, "wherein people strive to show how moral they are by thinking... even more lowly of standard exemplars of bad..." Certainly possible, and plausible.
But there is an alternative explanation: Hitler is the Devil — for Protestant atheists (secular progressives, in the cladistics of Mencius Moldbug). And why Hitler, of all the terrible people who could be elevated to Devil? (Note Hanson's theory does not explain this.)
The theory of Protestant atheism has more explanatory traction here. Democracy and industrialism are arguably the two major dimensions of Modernity, and Modernity is a bargain with the actual Devil. Hitler is perhaps the purest, the least alloyed product of industrialism and democracy, before Modernity evolved its outer armor involving several layers of confusion and obfuscation. Hitler may be a uniquely dramatic embodiment of everything that is wrong with Modernity, but there is no way to say so without endorsing an essentially Christian eschatology. The problem is that people don't want to be Christian; it's pretty much mutually-exclusive with cosmopolitan success via symbol-manipulating careers. However, they still want to say that bad things are bad, and that some things are so bad that they're... really bad. So they must, ultimately, generate a symbol of the Devil. That is, they must eventually believe in the existence of the Devil. And what symbol will they converge on, if not the explicitly theological one that's been on offer for ages? Well, whatever is too much themselves, whatever dramatizes their own bargain too clearly.