@skytempest: Too many possibilities really.
1. To hide her from his enemies.
2. To protect her from the influences or these supernatural beings until she's old enough.
3. To have her raised among humans who'll know how to actually handle humans (instead of the 'let's have her eat carrot/fairy grasses'...)
As for why he's left her alone this long, maybe he can't enter the human area? Wouldn't be unusual for the king to ask him to stay out since even a minor temper flare from him can result in many dead. (see: that other otome game manga about villainess capturing the last boss, with its own 'pokerface but actually very much want to dote on his waifu' guy)