Because the game is completely redone from ground up, using custom coding and his own assets Nintendo can't touch it. If he used any of Nintendos stuff then he would get into legal issues.
More than 25 years after its release, The Legend of Zelda: Ocarina of Time still tops Metacritic's list of highest-rated games. Now, an ambitious fan is reviving it with a remake in Unreal Engine 5, and a playable demo is already available.