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.
Even Supernatural has actual ending after 15 seasons
They had two! One at the end of season 5 and then at the end of season 15.