That has a lot more to do with the fact that the best running backs often go to college when they are draft eligible, or 3 years. That being said, there is both a positive and negative associated with a college RBs workload and it depends on preference. Low miles = less injury risk, but low experience. High miles = More experienced, but higher injury risk. When it comes to the 1st round, and even 2nd round, you want a prototypical balance, but you'll often find that most guys with more carries are taken sooner because not only does experience matter, there is more film to better project his progression from college to the NFL.