They’re both so universally well-known each man became synonymous with a word. For Einstein, that word is genius. For Hitler, that word is evil. They’re archetypes. Do you want to write a fictional novel, and have a character in it who’s a wide-eyed, messy-haired scientist, someone with a quick wit, a great mind, easily distracted by his surroundings? It’s hard not to see some similarities with Einstein.
Likewise, if you have a dark overlord ruling over some empire with goons and minions doing his bidding, there are camps, there’s murder and oppression, and the state is totalitarian… again, it’s hard not to see similarities with Hitler.
Einstein pretty much just means smart, and clever. Hitler just means evil, twisted. Of course, the two men were more complex than this, but it pretty much boils down to that. I’d say both are equally well-known as everyone has some sort of feeling about them and this is the most common one.