Here's why:
* The Elder Wand: Voldemort believed he had control of the Elder Wand, making him invincible. But Harry had unknowingly become the true master of the wand by disarming Draco Malfoy, who had disarmed Dumbledore. So, the Elder Wand refused to kill its true master, Harry.
* The Love Protection: Voldemort's killing curse backfired because Harry was protected by Lily's love, which was a sacrifice made to save him. This love-based protection had been working since the first attack on Harry when he was a baby.
* Voldemort's hubris: Voldemort was ultimately defeated because of his own arrogance and belief in his own power. He underestimated Harry and the power of love, which ultimately led to his downfall.
Though the fight was intense and both wizards were powerful, Harry's victory was ultimately a result of a combination of factors including the Elder Wand, Lily's love, and Voldemort's own mistakes.