NFL teams ask their running backs to do more than just carry the football. They also have to be able to contribute in the passing game, both as receivers out of the backfield and in pass protection.
Getting up to speed in that latter area can be a challenge for players entering the pro level from the college game. The NFL is a lot more physical as well as sophisticated; being able to identify blocking assignments and then carrying them out properly is not an easy task to do for players who might have little to no experience in this area.