I don't have the data, but I do thing there is something too using the fullback more often. Minnesota lines up with a fullback more often than any other team in the NFL, and lo and behold they are second in the league in total rushing, behind Baltimore. Dallas doesn't use the fullback very much and while they still rank highly in total rushing, it seems like they really limit themselves by not using a fullback. I understand that modern offense minds don't like using a fullback because it limits their offensive personnel, but some teams should consider using it more often, especially in run-heavy downs.
This is similar to a related problem I have with teams doing so many run plays out of the shotgun, especially around the goal line.