I think there are a number of variables in the mix, and since we're sitting on the outside, we'll never really know how and why things went down like they did. And Kuwagaton said, this is a thing where we just throw bile and hate in all directions, and maybe (certainly) we do it more than we should, and more than the films actually deserve. None of them are perfect, but I don't think MoS or even Squad is any more flawed than the average Marvel film. The DCEU just isn't allowed the wiggle room the MCU gets.
However! I am a fan, and as a fan I blame a whole number of things and people.
First, I do blame whoever it is who is in charge of WB's DC movies. We've seen tons of problems behind the scenes; changes in management and administration and cast and director and other behind-the-scenes guys, films being green lit when there isn't even a script or director attached, films announced then delayed or cancelled, no one knows whether Affleck is playing Batman this week or not, now Cavill is probably out as Superman but he's still got a contract to fulfill so who knows how that'll end.....there's chaos in the building and no one seems to have a foundation under them. At least, that's how it sounds from here.
I also think they tried too hard to establish a shared universe too quickly, worrying about the next film or two down the line rather than focusing on the film in front of them. When you can't divert your attention to your current project, odds are people won't care about the next one.
I don't think WB's decision makers really get Superman's appeal. They understand Batman; he's a simple archetype that is aimed at appealing to the lowest common denominator. Everyone has had a bad day and wanted to take it out on those we perceive as responsible. And all you need to play Batman is a good looking guy who can growl and spend six months in the gym before filming. Superman's trickier, and he sort of demands that the audience believe in something better. Superman requires a little faith, but WB seems to miss that, and therefore they hire the wrong people to oversee the character.
On top of that, the public has a very strong notion of what Superman is, even though they really have no idea of who Superman actually is. And fans, we're even worse. And because Superman is Superman, he gets put under the microscope more than any other superhero. He's the first, he's the best, and he damn well better fit inside the box people have for him. If he doesn't? Hellfire and brimstone. Superman isn't a character who gets to be a character, he's an ideal in people's heads (that he never really completely represented, if you read the actual books) and he has to fulfill the Space Messiah archetype we put on him.
I also think there's a social component. Superman is about restraint. He could rule the world. Kill every major villain on earth. Be rich. Powerful. But he works a 9-5, gets yelled at by his boss, and is barely middle class. "Restraint" is something people seem to have forgotten about and abandoned.
Marvel isn't helping either. I think a lot of people now look at the MCU as the blueprint for doing a good, traditional superhero movie. Sure, you can get away with the occasional Logan or Deadpool, but by and large the public now thinks that a spandex superhero movie should be fun, intellectually lite, and bombastic. Man of Steel's more dramatic, serious take was maybe a few years too soon, and starring a character few people connect to "dramatic," and I think it caught everyone off guard and the DCEU never provided the quality people needed to recover.