The OC's been tanking it this season in the ratings, and it looks relatively expensive to shoot.
Obviously, the ratings free-fell this season (that happens when the magic of the show dies and it becomes Melrose Place 2.0) but friday nights would basically kill the remaining fanbase it has.
The O.C never recovered from the mistakes it made in the middle of season 2. OC suffered from the rare case of the "perfect" first season. It proabably would have been better ending the run right there because it ended perfectly. Adding 2 (and now 3) more messy and cliche loaded seasons really just hurt the legacy it had going for itself.