Why did Tim Burton stop directing the original Batman films and get replaced by someone else?

Tim Burton did not stop directing the Batman films. He directed the first two installments in the series ("Batman" in 1989 and "Batman Returns" in 1992) before being replaced by Joel Schumacher for "Batman Forever" in 1995. It was Burton's decision to leave the series, as he felt he had explored the character fully in the first two films and wanted to pursue other projects.