Because I like Simpsons.
Also, due to the location, I really have no idea what they would replace it with. Tearing down Simpsons/Back to the Future and replacing it with like a stunt show (someone brought this up prior) would be a huge downgrade in me eyes when Water World is already in the park.
Like, yeah Simpsons is probably leaving sooner than later, but I reallly don't want it to, haha. I get a bigger kick out of Simpsons than I do Mummy believe it or not.