Short version: Whedon was on a hiding to nothing, trying to fit two quarts into a pint pot.
This seems to be the story from rumours: WB saw the success of the MCU, but didn't want to put the work in. Justice League was originally supposed to be four movies with backstory and character buildups (as per the MCU movies like Iron Man that came together with others as the Avengers movies). WB told Snyder he could only have two movies. Then he was told one movie that couldn't be more than two and a half hours long. This was never going to work, and the relationship between the studio and Snyder deteriorated. It's not known for sure if Snyder was sacked first, or if he left after the suicide of his daughter to be with his family.
Joss Whedon was brought in to "do an Avengers" to try and fix the movie, while staying within the WB time constraints. Too much humour, story and characters cut back, massive reshoots, crappy CGI and the infamous disappearance of Henry Cavill's moustache. This explained the poor theatrical version, as the studio tried to stuff four films into one at a very limited running time.
This brought the fan campaign to bring back he full Snyder cut version. WB needed something big to draw people to their HBO Max streaming service. All interest in Game Of Thrones (which was their big jewel in the crown) died after the terrible last season, and took the planned spin-offs with it. With nothing in production due to Covid-19, WB got Snyder back to finish the film he wanted, with the runtime restrictions removed due to being on a streaming service. AT&T owns WB, and is in massive debt and may have forced WB and HBO Max to pull this rabbit our of the hat.
This brings an interesting situation. Before the success of the Snyder Cut, Justice League's failure was being used as a way to kill off the Snyder-verse version of the DCU. JJ Abrams and his wife have been brought in on a massive $500 million contract to build the DCU (he will destroy the franchise with his overt SJW agendas like he did with Star Wars and Star Trek). Now the Synder cut seems to have been very successful, and there's a possibility of the Snyder-verse continuing alongside whatever Abrams wants to do, but this would be in direct conflict with Abrams' plans. AT&T want money to clear their debts first and foremost, and will push WB and HBO to do whatever it takes to make money, so must be eyeing up the success of Justice League and wonder if that's the path to follow.
Watch this space to see what happens when JJ Abrams screws things up, and what happens with James Gunn's (Guardians of the Galaxy) Suicide Squad 2. There's even unlikely talk of a David Ayer cut of Suicide Squad so he can get his original vision, rather than the poor version with the excessive interference from the WB studio.
How did AT&T become so much in dept after they just purchased Warner, Maybe they will end up selling and someone who know something can take over, its been a bit poor since they took over, they just want everything to be quick and expect it to make money, which does not happen.
Ok So its was a f up partnership.
They Wanted to move away from what DC is.
This is why they are failing, spending money to bring in new guys, reshoots, and not checking the work, If they just spent a little time spreading the movies across a few years, they could have done much better,
If they did a part one then in 6 months later did a side movie and 12 months later did part 2 it would have been better, just look at LOTR, Marvel, or even better built it up over a number of years, we had not info on anyone of the heros other then superman and batman. I think if they built up before BvS maybe had a at least 1 batman movie. IMO they had no real idea, They had a ready built model with Marvel and Star wars, but just did not want to put in the time.
I mean DC has better more in-depth characters and storylines if done right marvel cant touch them but they cant.
They even had a mostly all start cast unlike marvel who had unknows at start.
also JJ Abrams and his wife, first of all is JJ is he not the Sci-Fi guy does he know much about super heros and what does his wife do to be brought in.
I would rather have a comic book guy for creative like Kevin Smith.
christopher nolen would be a good option to direct we know what he did with batman but he's a little stuck in his ways. also maybe the Russo Brothers.