Ok, we're talking all time elite RB's here. Guys like Bradshaw, Lynch and Benson are nowhere near that level.
Durability was brought up. Part of being durable is training hard, knowing how to take care of your body, and having the discipline to do proper rehab when necessary. Hard working, high character, high intelligence RB's know how to take care of their bodies and increase their chances of having long careers.
I don't agree that all positions have the same correlation of elite players having high character and intelligence. I can't think of any RB I think of as elite getting into any trouble during his career. I see guys like Lawrence Taylor and Michael Irvin who got in major legal trouble while playing, but were still elite.
Oh, and Emmitt Smith was a smart football player. He may not be good as a speaker, but he has a good head for competing.