Well, well, interesting question. In Melee, where Zelda was introduced to the series, the transformation tactic was one of Zelda's most important abilities. In Brawl, Sakurai wanted to keep the original concept of Zelda as a fighter, but with the improved graphic capacity of the Wii, the Ocarina of Time version of Zelda would have looked goofy. As such, he decided to go with the Twilight Princess model. Interestingly, Shiek as he (while biologically female, the entire point of his existence is for Zelda to be disguised as a male Shiekah) appears in Brawl is also not from the N64 games, but is instead an unused concept from Twilight Princess.
Sheik was going to be in TP, but she was cut from it for unknown reasons.
Guessing fans did not want Twilight Princess 2 ( but only playing as zelda and sheik!!!) :)