There's been reasons for that. Not to make excuses for it. Part of it was due to injuries. Julius Jones and Demarco both should've had 1,000 yards multiple times if it wasn't for injury. Specifically in both of their rookie years. Also part of it was we've always had a running back by committee. There was one year where it was Tashard, Marion and Felix. A few more years where it was Felix and Demarco or Marion and Felix. There was Julius and Marion as well.
You have to look deeper.