Season 1s are great, setup, some payoff, a bit of lead into the overarching story. Then season 2 to X. The heroes win and then lose in the final episode, cliffhanger to next season. People get bored. Final season is announced and they wrap up the show.
I don’t look at it like an overarching plot so much as anthology. Character A and Character B have chemistry and should be together but it doesn’t happen. It just happens that there are several stories that involve that failure.