Otogakure was removed because of the addition of Iwa and Kumo. The thought was to only have the five main villages as a playable race.
However, if you look at Neji's comment in this thread, he has willingness to put it back but he wants a safe haven from pvp in the game.
So if you really want Oto back, you have to convince Neji to move the safe haven pvp elsewhere, and then he might put Oto back since I'm fairly sure it would be easy enough to code and such, since it was apart of the game at one point.