I think the core of the problem is that like a lot of directors, Johnson didn't want to do someone else's work. He had JJ/Kasdan TFA before his movie, the Kasdan/Treverrow TLJ script already done, but he wanted to do his own thing, make his own name, and Disney were dumb enough to let him throw out what went before.
It's like he said "oh, don't want to do the Snoke storyline, kill him off, don't want to deal with Rey's manifest destiny story, bin that, don't want to do Luke as a messianic figure, throw that away, don't want to do Finn mortally injured, make him recover with no ill-effects in the first five minutes. Guardians of the Galaxy crossed with the new Ghostbusters? Michael Bay style with a Star Wars theme? Children's toys and merchandising? That's the way to go!"