The main thing was that Black Adam didn't perform the way DC and Warner Bros hoped. The movie came out in 2022 and honestly underperformed at the box office compared to what they'd invested in it. When a big-budget film doesn't hit those numbers, studios tend to pump the brakes on sequels and spinoffs, especially in a shared universe that was already pretty chaotic at that point. Johnson had come in with this whole plan to shake up the DCEU, but the movie just didn't give them the foundation they needed to build on that vision.
Beyond the numbers though, the entire DCEU was kind of in flux around that time. Warner Bros was going through major leadership changes and they weren't sure what direction they wanted to take with DC projects overall. Johnson wasn't going to stick around in a universe that didn't have clear plans or momentum, especially when his time and brand are valuable. It wasn't really drama in the tabloid sense - more like a business reality where both sides realized the partnership wasn't working out as planned. Sometimes these things just don't click, and that was one of those situations.