I think there is a concern that the actors don't want to commit their entire career to a single project. They become popular because of this show and they want to cash in doing other work. Or they want to be known for more than just this.
I'm a huge fan of Deadwood. A hugely underrated show that only ran 3 seasons because they couldn't get enough of the cast to agree to terms for the fourth season. After 10 years they are going to wrap up one of the best series that has ever been on television with a feature length movie (May 31st on HBO). I can't tell you how horrible it was to not get season 4 of that show.
There has never been a show that had better casting than Deadwood. NEVER. The writing for that show (however filled with cursing) is pure poetry. The only thing wrong with Deadwood is that it wasn't finished, and it wasn't finished because the cast wouldn't commit to more seasons.
I have low expectations for the Deadwood movie, but I'll be happy to get a conclusion ten years later.