Marketability aside, I think one reason we keep getting such a disproportionate amount of Batman media is because in general, and especially compared to Superman/Wonder Woman/Aquaman/etc, Batman’s mythos is very low-concept. Sure, occasionally you have some good old fashioned comic book nonsense like Poison Ivy or Ra’s al Ghul. But the most prominent villains-people like Joker, Riddler, Two-Face, Scarecrow, and Penguin-are basically just regular criminals with flashy outfits and elaborate ways of doing their crimes.

This is not to say that Batman is better than the other heroes, just a lot easier to adapt (especially for the Christopher Nolans of the world who want to do ~realistic~ comic book adaptations)