What happened to the Ropers on Three's Company? Why did they leave the sitcom?

So they could star on their own sitcom, The Ropers.

The British version of Three's Company, Man About the House spun off that show's equivalent to the Ropers, into a show called George and Mildred (https://en.wikipedia.org/wiki/George_and_Mildred). So there was already a precedent being set.

Plus, Three's Company was probably at the peak of its popularity at the time, so I would figure that spinning off arguably the two big breakout characters would result in similarly huge ratings. ABC around this same time, had numerous spin-offs to its other big sitcom of the time, Happy Days such as Laverne and Shirley and Mork and Mindy.

And CBS had spun Maude and by extension, Good Times and The Jeffersons off of its biggest sitcom of the time, All in the Family.