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 like the idea that a lot of series are repeating Act II over and over. I had never thought of it that way, but it makes a lot of sense.