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.
Look at Black Mirror. The British seasons are some of the best TV ever made. The American (Netflix) seasons have often been meh or downright awful, and derivative of the original seasons.