I believe it can sometime be a matter of contracts with the studio producing the series.
My understanding is that most show runners start with a 1 year contract, which can then be extended in two year increments by the studio depending on the shows ratings and popularity. So season 3,5 & 7 can often end with a major change up behind the scenes with can then lead to good shows going downhill in either season 4 or 6 (and maybe 8, but most shows never make it that far). the actors are usually on different contract lengths which is why they sometimes leave part way through later seasons, or have to stay on even after the original shows creators have moved on. The same goes with writers who usually only have 1 year contracts, so over the course of a series a lot of the original writers might be eventually let go so you are left with people writing the show that wern't on it at the beginning.
I also feel that 5 years is probably the most time you can expect a a show creator/runner to maintain a decent amount of interest and enthusiasm for the shows storylines, characters, concepts, setting, etc... These people are artists (usually writers that have moved up the Hollywood chain) and five years is a long time to be focused on one set of characters.
Supernatural is a show facing this dilemma at the end of this season. the shows creator's contract is up so he is leaving (after 5 seasons), but the main actors are all still contracted out for at least one more season, so if the studio/network decide to do another season, they will have to get a new show runner to pick up the reigns and take things in his own direction. Whether this is good or bad depends on what he decides to do with the show.