Just overextend their welcome. Part of good shows is the idea of leaving the audience wanting more. I remember watching a documentary on Netflix about TV shows by Tom Hanks. It had the showrunners of Lost being interviewed about it and when ABC ordered more seasons the showrunners were like oh crap where do we take this show. They clearly didn't have a plan on where to take the show.

I think though networks like to wring shows dry. Trying to squeeze ratings boosting plot to line their pockets. Part of a good show or just good content, in general, is knowing when to quit.

Especially in this day in age with online streaming. I would be ok with TV shows lasting 1-3 seasons and being done.