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.
Eh, Scrubs suffered a lot from trying to find ways of keeping the characters somewhat together and on the hospital. Also, even if not relevant to the topic, flanderization utterly fucked Scrubs from season 5ish onwards.