The running back position has been phased out somewhat over the last few seasons. Whether it be the prevalence of pass-first teams, or teams using multiple RBs to carry the load. Teams just aren't spending first round picks on RBs quite the way they used to.
Ryan Mathews was the first RB taken in 2010 and had a disappointing season. While injury played a part in that, teams will look at Mathews as an example of why they can find productive RBs in later rounds.