The reason is probably because of violence, blood (as seen when Link strikes Dark Beast Ganon it drips when he runs around, and also when a Deku Baba is defeated), and signs of nudity (as shown by the Great Fairy, although only violence and blood are listed as descriptors for the rating).
This is the first Zelda game to be rated T by ESRB and 12+ by PEGI. It was highly anticipated by many members of the gaming community and was regarded as finally fulfilling the dreams of those who wanted a much more realistic and mature Zelda game, as seen in the SpaceWorld 2000 GameCube Tech Demo. The Legend of Zelda: Twilight Princess is the thirteenth main installment of The Legend of Zelda series, released for both the Nintendo GameCube and Wii. Twilight Princess Guide at Zelda Universe The Legend of Zelda: Twilight Princess guide
