He was originally projected in the 2nd round, so slipping to the top of the 3rd round was not much of a slide. I think his not getting picked sooner, was more about teams evaluating value at positions of needs at the time of their pick, etc.
He was already pretty productive at Arkansas, but I believe he is still considered a raw prospect in many ways who needs to continue to develop counter moves, get comfortable with his recently added weight etc. It is his potential upside that makes people say he is a great value in the 3rd round, because If he hits his full potential, he would be a first round player.
We all know that there is no guarantee that he will reach that potential, but even his current production (6.5 sacks last year, 2nd team all SEC etc) combined with his elite combine performance, make him worth the pick, in my opinion. It is if and when he gets closer to his ceiling that he will become a true "steal".