I just don't think we know if that is the case. He's a straight-line runner with power and speed. Maybe that would have been enough for some success behind this line, but it would have limited the running game's effectiveness IMO. But that's all speculation. What we know of him right now is that he hasn't gotten on the field as a running back yet in a regular-season game, and it's possible he never will or it's possible he'll become what you think he can become.
I don't like to play with projections because most of the time it turns into a grass-is-greener view of a player. Jackson is better than Dunbar in some fans' minds because they've seen enough of Dunbar to know what he is and assume ability with Jackson instead of believing that the coaching staff who saw him every day didn't see enough to keep him.
I'm not saying the coaching staff couldn't be wrong, just that there is no reason to assume Jackson is more than just the guy that they didn't think showed enough to knock a backup with a one-year deal off the roster.