Because there are enough other missing pieces on the team in this transition year that we may not gain much from playing him. Perhaps, even though he is the better guy saving him makes the most sense in the long run.
The ceiling isn't that very this year. Transition year. New coach in every position. Really tough schedule.
I would understand either way Frost goes here. Get the best guy in place and play him, or teach him up and save some eligibility when he is closer to his prime. It's not hard logic to follow, really. Just depends what Frost wants to do.