I think people harp on/overrate relatability in this kind of thing.

after all, we are discussing this in a BATMAN thread. the guy who makes Tony Stark look relatively tame in terms of how freaking ridiculous and far from the common man he is:
*insert billionaire playboy philanthropist list difference joke here*

you can't discount charisma/style in this kind of thing. sure a stand up guy with relatable common man backstory is safe, but they can also be plain. Batman? has that dark mysterious brooding charisma he can stoically do the quiet badass thing of being this inevitable doom for the villains but also can have his moments of caring for his bat-family or friends, Tony Stark has this egotistical Vegeta-like attitude of "hold my beer I'm going to do something awesome" which is always fun because its always a coin flip whether he actually succeeds and proves that he is not all talk or fails and is taken down a peg and you enjoy it either way.

escapism can also be a factor: sure not everyone is rich, but people sure do want to BE rich and if you tie in that wealth fantasy with a bunch of spy gadgets and cool tech to fight badguys that can lead to a wealth fantasy of using the money for good in cool way rather than a boring one.

while Superman? can't really feed into the fun part of being a superhero because he is too busy being responsible with it. sure he is a good guy for being so responsible, but it can also make him come across as stiff because if your in his position you can't really have any fun with said powers if your held to such high standards. not really feeding into the escapist fantasy.