Originally Posted by LeoAmes
I'm not sure, but I assume the ratings declined enough to make a full time production not worthwhile, but still good enough for the occasional movie to garner attention and bring back the fans that might've stopped tuning it every single week during the latter seasons of the show.
A few movies after the regular end of production seemed to be a somewhat popular way to send off shows in the latter 70s and early 80s. Two of my other favorites (Little House on the Prairie and The Waltons) did something similar 5 years later or so, and there are other examples as well.
I thought i read somewhere that the cost of show was getting too high, so, they decided to cancel and/or not renewed. I also think 6 years is a pretty good run time and that maybe it was time to end the show.
I am glad to have S1-S6 on dvd.