It was only after getting the secret stone back from Ganondorf's body did Zelda realize she had the power to save Revali.
She held up the stone of her ancient Zonai predecessor, shining in all its tear-shaped gold through the darkness of Hyrule Castle's depths. She was alone at the site of Link and Ganondorf's battle, her hero having gone off to hunt down the exact monsters who'd helped kill the rest of the Champions. So gripped was he by his justice-combined-with-revenge spree that he failed to notice the hyper delirious state Zelda was in. The trauma of seeing Revali cut in half with his blood spraying all over her had cracked something in her.
She had only Revali in her mind as she stood up in that dark, death-stenched chamber, gazing down at the stone in her hand. Not even Ganondorf's grotesque death grimace, with its gaping mouth and white, rolled-back eyes glaring up at her could break her from her trance. The only thing Hyrule's princess saw was the divine stone glowing in her palm and Revali when he was alive.
Zelda vanished in a glitter of golden-white light, taking the secret stone with her.