Ending of The Big Bang Theory

It was Parsons' decision to leave that finally ended the show.

OP's reference

“What had occurred was Jim Parsons had been in touch with Chuck Lorre over the hiatus week and had said he couldn’t come back and do anymore. And Chuck had always said if one member of the cast left the show then the show would have to end."

Radio Times

The Big Bang Theory‘s Jim Parsons has revealed that he decided to leave the long-running show after experiencing “a really intense summer” before filming season 12.

Speaking to David Tennant Does a Podcast With…, Parsons, who played Sheldon Cooper on the sitcom, explained how he realised that there were “other things [he] needed to try and do” after a chain of upsetting incidents happened in his life.

“Our final contract was for the last two years, but no one knew when we signed it what that would mean,” he said. “I kind of had a suspicion in my heart that that was going to be it for me when I did sign that contract.”

He said that his feelings of wanting to leave were affirmed after the Emmy-winning actor became overwhelmed with filming the show, rehearsal for a play, appearing in an Intel commercial while his dog’s health was declining. “I was exhausted. I was really upset about, more than anything, one of our dogs was getting really at the end of his life around then.”

Parsons added that a few days after his dog was put down, he slipped and broke his foot whilst on stage. “It was the scariest moment for the next couple of days because I felt like I was at the edge of a cliff. I was teetering and I saw something really dark below between the death of the dog.”