People want a good story and a charismatic performance from the actor playing Superman, which they didn't get. When Antony Starr is giving a much more charismatic performance as an evil piece of shit like Homelander than the actor playing actual Superman we have a problem.
So tone being the main issue continues to be a lame deflection. General audiences and fans can accept Superman in a darker story if the execution is done well, it simply wasn't here. If you're going against the grain of the "rules" of Superman (or any established character), you need to make damn sure you replace it with something fresh and interesting, then people will be won over. "Subverting expectations" or "trying something new" doesn't automatically get a free pass for intent if the execution is sloppy or wooden.