The O.C. was getting tedious in Season 3. Viewers took note of it and the numbers started dwindling, but the decision to kill of Marissa Cooper was the thing that pissed viewers off the most. Season 4 was surprisingly great, but it was clear people were only watching for Marissa and the show was cancelled after a shorter than normal season.
Realistically, how many times can one person escape from prison? Viewers asked themselves that question many a time during the four-season run of the Fox series. It had its great moments, but the concept was just not fit to be told over a long arc. Season 4 was all over the place, with Sara returning from the dead. And now Michael is returning from the dead in a rebooted series next year. Can we all assume the show is supernatural, or something?