Nats management is claiming it's for his own good, but it has more to do with an baseball rule regarding players contracts. The Nats could lose a lot of money if they bring him up too early.
If I can find the article again, it explains the incentive to keep the best prospects in the minors.
If he is not brought up until June (assuming he doesn't have some freaky regression), it is clearly financial. He wasn't started immediately, because he's never really faced anything but college guys and needs to get acclimated to playing professionally. He could have been brought up immediately, but it is best to bring a guy up when he is prepared to succeed and stay up. After about 5 starts in the minors, given how lame our pitching staff is, there will likely be little actual baseball reason to not bringing him up to develop in the bigs. There's no reason to rush him if he is not ready. But he'll likely be ready before the June timeframe. The organization wants to control the situation financially, and be ready to showcase the kid for maximum marketing pull. Consequently, he likely won't be up until they had him pitch in the minors for an extended period.