Nah, it's probably more a matter of the contract that the cast signs for the pilot generally running for about five years (relatively well-known names like Caan and Cox probably being able to negotiate shorter contracts with built-in raises), at which point every series has to start either...
Not so much canceled as reaching the end of its run. It's at 83 episodes right now; another 17-episode season will get it to 100, which is used as the benchmark for a show being appealing in third-run sales (probably to USA and Universal HD). It's probably a nice number for DVD sales, too -...