Only thing I can think of is that he's done with school and wants to test the FA market. Probably didn't feel like he would play much in 2016.
Even if he didn't play much (and that's very arguable- if he stays healthy, he would have played as much as anyone IMO), He would have helped himself just by proving he can make it through an entire season. He can't hurt his draft status by staying, because he HAS no draft status. He can't go any lower. By staying, he literally has nowhere to go but up.
I'm not mad because we're losing him. He's barely even played the past two seasons. But on a personal level, I think he's hurting himself with this decision.