I heard, the real reason for this is the success that it's gained was it's downfall. With minimal pay increases for the cast, after a certain period of time the show can no longer pay the cast and still make money, no matter how much merchidising that you do. 

There are rumors this is the real reason RDA left - to give his costars another year with out the show running in the red. Of coarse those are just the rumors, but it's nice to think that he liked the rest of the cast enough to do something like that.

After 10 years they'd have to totally revamp the crew in order to continue on, the producers thought it better to just launch a spin off instead of forcing the fans to except a totally new cast. I'd like to see Claudia or Ben to get picked up on a second spin off of the series.