https://www.metv.com/stories/carol-burnett-admitted-the-reason-why-she-ended-the-carol-burnett-show-after-11-seasons

The actor wanted to leave on her own terms.

https://www.metv.com/stories/carol-burnett-admitted-the-reason-why-she-ended-the-carol-burnett-show-after-11-seasons

The actor wanted to leave on her own terms.

For CBS The Carol Burnett Show was one of their crown jewels so handling the end of the series had to be done in the most respectful way possible. With that as the set-up let's take a look at the METV article.

First a housekeeping issue, this information is incorrect:

"Still, after more than a decade on air, Burnett decided that the series would end in 1973".

The show ended in 1978!

The Burnett show had fallen on hard times by the time the series ended its run, so Carol's comments are not credible:

"Ratings do not have a thing to do with my decision," said Burnett. "They have been much lower before. And CBS picked the show up for a 12th year...But it's classier to leave before you're asked to go. I'm proud of our show. Now is the time to put it to bed."

The show had a big drop in popularity in season 10 falling from 28 to 44 in the ratings. With the exit of Harvey Korman (I think there's more to that than meets the eye, but that's a topic for another day), CBS pressured Carol to add Dick Van Dyke to the show in hopes that the drastic rating decline might be reversed. Of course the move didn't help in fact the ratings continued dropping the show finished season 11 at position 66. There's no way CBS was renewing an expensive variety show performing that poorly. I'm sure they went to Carol offering her the opportunity to announce she was ending the show rather than cancelling it.