If you can't hold onto the ball, you don't get to play. Crowell not only fumbled multiple times, he allowed himself to be quoted saying he doesn't have a ball security problem. Crowell is not a very bright kid and his other quotes about hoping he will play more, how hard it is to get into a rhythm while sharing carries, etc., have not gone over well with the coaching staff.
Accordingly his snaps are disappearing. Its not that surprising, really.
From Matt Waldman's Gut Check:
The typical water cooler conversation Monday morning was
Isaiah Crowell looks good, but he needs to work on his ball security. This is outcome-oriented thinking that stems from seeing three fumbles in the Steelers' contest without understanding the context of the plays. Considering how much NFL coaches dislike fumbles, if Crowell's fumbles were the way the general public were reacting to them do you even think he'd still be on the field after the third fumble, much less the second?
Of course not, and the reason Crowell still earned the ball was because the fumbles in this game were a) unusual and b) not egregious issues. Crowell dropped a pitch that constituted his first fumble. It was a clear mishap that the runner pounced on and then got up and turned a potential loss into a two-yard gain. The second "dropped pitch" was a poor exchange with the quarterback.
When a veteran quarterback like Trent Green, who was the analyst on this game's broadcast, noted that the pitch was not good and made the play difficult on Crowell, it's worth taking into account. The third play was a true "forced fumble," on a run where the defensive back delivered a perfect hit on the ball with a downhill head start. This is the type of hit that would have jarred the ball loose in 9 out of 10 carries by any NFL runner.
If this wasn't the case, Cleveland wouldn't have put Crowell back into the lineup and given him the ball a few plays later -- especially not after the mistakes that occured with the pitches.