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.
Scrubs had its problems too. It just didn’t matter as much because its a comedy first.
For example: Elliott took like 6 seasons to gain confidence. Probably because JD kept trying to get with her then being an idiot and breaking up.