Entertainment Weekly says that JIM PARSONS wanted out and the producers decided there wasn't a show without Dr. Sheldon Cooper. That's why CBS said that the show would be brought to an end for "creative reasons."
There's no official word on exactly why he wanted out, but it’s been speculated that he just wanted to do something different. Jim did post a long message on Instagram (below.) He didn't address wanting to leave but he DID say he was grateful to the cast, crew, writers, and fans of the show.