I think Luck proving he could throw the deep ball is what energized the run game most. Also, Mack came back at the same time TY Hilton did, which meant the running game faced less pressure. I'm not saying Mack isn't a solid runner, he is, but he's not light years better than the other guys. Jordan Wilkins averaged almost a full yard more per carry, he was an effective rusher, he just had a fumbling issue early in the year, which very well could be fixed now.
The o-line improved as the year went on as well, especially once Kelly got healthy. With the exception of the wildcard game against Houston, almost every game where Mack had success, the other RB's had higher YPC than Mack did, just less work. Maybe they feel Mack is a much better player, maybe they didn't want to put too much on the rookie's plate, and maybe they didn't want to fix what wasn't broken, but my point is, Mack may or may not be the best RB on the team, and I have enough concern about it becoming a RBBC, that Mack will likely be around RB20 for me.