It hit me after looking at the video of Derrick Henry working out. He is one special athlete -- but RB's are a dime a dozen and not valued by the NFL. Look at how many WR's went early compared to RB's. If I were DH and wanted to be a 1st round draft choice and play in the NFL for 10 years, I would immediately switch to a rush DE. With his speed, power and quickness he would be impressive...in the pass happy NFL no one needs or wants RB's or "big" DE's. They want quickness and speed...
Tailbacks have more value in the college game, to be sure, but even there it's not really a hard position to fill. You'll find absolute stars every now and again -- Peterson, McFadden, Richardson, etc. -- but for the most part there are 40-50 prep prospects every year who can get the job done in the right situation, and it's just a matter of signing a guy or two like that most every year and then putting him into a positive situation (i.e. good OL play and a decent passing game for protection). It's just not worthy of all of the hand-wringing that people experience with prep tailback recruiting.
And as for players who demand to be tailbacks, it's insane. It's the most physically punishing position on the field, it's the position with the shortest average career, and in terms of expected earnings over the course of a career, it's the least lucrative position on the field (below even kicker, punter, and, in some cases, long snapper).