It's puzzling, to say the least.
Not to suggest Alabama's rangy free safety Ha Ha Clinton-Dix isn't necessarily worth the 15th overall pick, but more why ESPN draft analyst Todd McShay doesn't seem to remember the previous mock draft he posted (behind a premium wall on ESPN Insider).
I'll paraphrase: McShay admits struggling with this selection, and admits one option might be to trade back, with nose tackle Louis Nix III being an option a few spots below the No. 15 spot. McShay doesn't like that option as much, though, because Nix's performance in 2013 didn't justify the No. 15 overall pick.
McShay had Nix going to the Steelers at the 15th spot in his last mock draft.
Not to get all dorky about this, but if you're going to do the job, do it right, Todd. Did his 2013 season somehow become better since the 2013 season ended? Maybe he saw something that changed his mind; that's fine, but write that. The flippant nature of his explanation doesn't serve his readers well.
Anyway...Clinton-Dix is probably considered more of a darkhorse for this position, and in reference to this particular mock draft, some might argue linebacker C.J. Mosley, cornerback Darqueze Dennard or tight end Eric Ebron might make for better value at the spot. At the same time, with the extension given to Heath Miller, tight end may not be any better of an option than safety - where Troy Polamalu also received an extension.