Also, the success of the running game is much more dependent on the offensive line than it is on the running back. An adequate running back with a good offensive line is much more likely to have success than a great running back with a weak offensive line. If a team wants to run, it makes more sense to invest early round picks on offensive linemen.
Emmitt Smith is a perfect example. I am a Dallas Cowboys fan and love Emmitt, but on draft day, everyone was saying:
Too small for the NFL.
Too slow for the NFL.
Too fragile for the NFL.
However, while running behind an offensive line with which my fat butt could have ran for 90 yards a game, he became the all-time rushing yards leader.
I am not taking anything away from Emmitt, he was a very SMART runner. He was a lot tougher than most gave him credit. He used that line very smartly and allowed them to clear the way, but of course, he was VERY quick to get into his second gear once he cleared the box.
I still remember on draft day how everyone was screaming that it was a wasted pick.