Warner Bros. had an enormous backlash after Batman Returns was deemed too scary for children. The Soccer Moms threw a fit at McDonald's because the happy meals had toys from "such an awful movie", so McDonald's threw a fit, so WB had to quietly hint at Burton that they didn't want him to do another Batman film.
Burton himself said they were making it plain they wanted him out of the director's chair, but not forcing him out.
Keaton was still on board if he liked the script. Keaton has said he read it, and didn't like it. He met with Schumacher. He told Schumacher what he didn't like about the script, and Schumacher wouldn't budge. Keaton says he actually wanted something similar to what Batman Begins ended up being, a film letting more into Batman's origins, if not an actual prequel.
Anyway, Keaton left over the creative differences, as he saw Forever was going to focus even less on Batman. Warner Bros. next offered Keaton 30 million to come back, but Mike still refused.
Sad, really. Burton/Keaton weren't keen on doing the second one, but once it was done, they both were excited about doing more Batman films, only to be shot down by the Soccer Moms of America.