Too unpredictable situation to keep him. Droughns was like 4th on the depth chart at the beginning of the season. Griffin got hurt and weren't Hearst and Bell also hurt when Droughns had his huge first game? Then Droughns had another huge game. After that, Shanny just decided to stick with the hot guy.
I honestly think Bell will emerge next year as the guy, problem with him is, he seems to get hurt alot. He's definitely got more quickness and speed than Droughns, one thing that helps the Denver RBs excel. He should come relatively cheap though since he hasn't played much at all.
And after 5 different guys have had monster seasons for Denver in less than a decade (Davis, Anderson, Gary, Portis, Droughns), Shanahan always has a guy he can put back there to get it done. I don't think Bell's any different.