If I were convinced that Najee Harris was going to be the best and most dominant RB in this draft I'd take him in the 1st round. But I'm not convinced it's him.
I'm convinced he's going to be one of the best - he's a lock pro because of his size, intelligence, and has no weakness other than lacking a top gear. But there's a tier of backs at the top consisting of about 4 or 5 backs that are all fairly close with different strengths and weaknesses. I just think your shot of landing the best back in the class is just as good at the top of the 2nd round as it is in the middle of the 1st round.
I think most teams typically see it that way in most drafts and it's why the run on backs usually starts in the 30's. A kid you like at RB is always going to be there in the 2nd round. More good backs have always been selected in the 2nd/3rd rounds than in the 1st round.
Occasionally there will be a draft where there's one back that teams believe sticks out as the total package that goes top 10 like Barkley, Gurley, Fournette, Adrian Peterson, Tomlinson, etc., but not very often.