I remember reading somewhere that in 1974, Flip's TV show was still scoring high ratings. However, he quit the series in order to spend more time with his children. I don't remember what my source was nor how accurate it was.
It actually slipped out of the top 30 in the 1973-74 season; variety shows in general were dying, with only Carol Burnett really a solid hit. Money was becoming a problem, too, as Flip's salary increased, and the show became more expensive to produce. I expect Flip said something like, We can do another season, but it will cost this much, and NBC said, No thanks.
Flip's Thursday at 8:00 time slot went to something called Sierra, which got pounded by The Waltons, and it lasted half a season.