As you already pointed out the Superman character development was in Man of Steel, BvS was a direct sequel, so he had no need for more of it and that's why there was so much focus on Bruce, although I feel like Snyder felt that everyone already knows all about who Batman is, so just to keep people from complaining "WTF, who is this pepper-haired guy in a flying rodent suit" he threw in some mention of his past leading up to now but not so much that made the other half of the viewers go "oh my god I know, I know! Everybody knows who Batman is! Get to the action bruh! More explosions and crashing through walls!!!" but I felt it was a good mix of "People know these guys, but I still want to flush them out and develop them for my film without boring the crowd with 65% of screentime being backstory and becoming who they are."
In short, you'll never please everyone, just like I posted on here a decade ago about Alien vs Predator, I'll say the same thing today about Batman vs Superman, they are both Gritty, yet with action, Crossover films that everybody has been begging to see on the big screen for ages, so that being said everyone has high expectations, however, look at how many people it has to please, while only being one film. They aren't going to make a super action packed, bloody, cussing, rap music playing, no story version for the dope heads that just want explosions and testosterone, then a version with jokes every 5 minutes and a bunch of standing around with your arms crossed and a bunch of fight scenes and fun old songs playing for the Marvel fanboys, then a serious dark tone film that delves into the emotional side for people like me who want an overly gritty film about two heroes I've loved my whole life, and then another with speeches about friendship and family with light piano tunes for the soccer moms who mistakenly took their ten year old to see BvS because Superheroes and child friendly is what Marvel conditioned her to expect. You can't please them all, and it's only one performance, so there will be mistakes. It's all a matter of opinion really, for instance, AvP, I loved it. Was it necessarily a good film? Perhaps not, it had really great aspects and a few dropped low-balls and "meh" points, so no, maybe it wasn't a fantastic film, I'll admit to that, but I still love it.
I personally just see quality in the film, and I highly enjoyed it, but it's hard to get something that big of a deal (C'mon, it's arguably the two most recognizable superheroes ever, third being like Spiderman, but the Batman fighting mother f'ing Superman) but something that big a deal juuuuust right the first (and possibly only other than sequels but I don't see a remake anytime soon) time in everyone's eyes. Just like I said about AvP like 12 years ago.