I have no idea why people, even people working at WB think Snyder's issue was that he has dark and serious. It wasn't.
Because that was literally what critics and normies told them. If I asked my parents about DC, they would ask why it is so dark and not funny like Marvel. This was the common sentiment during Zack’s reign and why Warner booted him and tried to MCU the universe.
As for why normies felt it, Zack’s shit was too long and overly complicated to follow. He tried to hard to be super smart with shit like Granny’s Peach Tea or the random Bible analogues. Not helping was how painfully stupid a good chunk of the plot was, best example being the nothingburger of Lois tracking down bullets in BvS.
The realistic tone clashed with the big spectacle action hard, making the fights lack a certain umph…
Man of Steel really suffers in its last battle from going way too big to a comical degree while not feeling like anything of substance has happened. Zack wants a big anime, DBZ smack-down but without the visual colors and general fun that brings. The miserable, 9/11 look only highlights how disastrous Superman is, and played a big part into why MoS’s destruction kept getting brought up even if the shattering of buildings is on par with the DCAU.
Finally, the mischaracterizations really bugged people. Batman killing was a huge point of contention, and before I hear about Burton, might I remind you those films came before the no kill rule was widely established in mainstream canon through Nolan and the cartoons. It is hard to walk back on 20-30 years of the rule now in play thanks to BTAS and Nolan being people’s defaults.
Superman lacking any fun was to his detriment and only solidified him further as a boring character. Cavil is good, but the script gives him nothing. He just sort-of looks stoic.
Maybe saying they were too dark is too general, but I get the sentiment. His films lacked a lot and felt too try hard without ever really getting to the point. Audiences just wanted the characters to lighten up a bit and not feel like they needed an hours long dissection of the plot to understand what was happening.