If he weighed about 20lbs heavier of muscle he would be the perfect style of RB for the modern NFL. A Marshall Faulk type hybrid RB/Slot WR.
But at his size, and considering NFL RBs are very replaceable nowadays, I can see why a team didn’t want to invest a pick on him and would rather take a chance on getting him as an udfa. He’s got too 3 round talent, but you can’t invest a top 3 round pick on a guy that might possibly be injured most of his likely short career because of his size.
I know it is a completely different position, but look at how rarely Bob Sanders was healthy in his career. He was a game changer when on the field, but missed more than half of his possible games because of injuries. And he was a brick house of muscle, just in a small body.