Why did they stop making Cheers shows? What was the reason?

Ted Danson didn't want to do the show anymore.

That's right. Ted Danson felt the show had run its course, and he was tired of the grind. Eleven years for any show is enough I guess.
I recall thinking at the time that Ted had some sort of mid-life thing going on.
All within the same year, he threw the towel in with Cheers, ended his long-term marriage and began a relationship with Whoopi Goldberg.
Here it is 10 years later, and he's obviously content. He's happy with his wife Mary and has a successful show in Becker.
I'm glad things turned out so well for him. He's one of my favorites.