1) He is a 3 down player.
2) He is a freaky athlete with unusual size.
3) He has all the same physical attributes and (as I recall) similar production as Calais Campbell.
4) This allows Brockers to move back inside as a NT where he was the most effective.
5) NTs are usually 2 down players and are much easier to find that guys like Tillery who can provide some outside pass rush along with strong run defense.
6) Rams just aren't that hard up for Oline to spend a 1st round pick on one.
7) My one caveat would be if a rush OLB like Sweat happened to drop into their laps at 31, then certainly he would have to jump ahead of Tillery.