I think he sort of pulled a Troy Hambrick. Hambrick looked good as a seldom-used backup to Emmitt Smith. Then, when Smith left and Hambrick was inexplicably made the starter, he flopped.
Smith thrived in a seldom-used role behind Elliott and Alfred Morris. Teams just didn't pay any attention to him because he was a third-stringer, so they didn't really know what to expect of him. He didn't get a lot more attention last year, but I believe defenses looked at him enough to know what to expect when he was in the game.