Here's why:
* The Elder Wand: Harry became the master of the Elder Wand after disarming Draco Malfoy, who had temporarily acquired it from Dumbledore. This made him the most powerful wizard in the world, at least in terms of raw magical power.
* His own abilities: Harry was already a skilled and powerful wizard with a natural talent for magic. He had proven himself in many battles, including against Voldemort.
* The Triwizard Tournament: Harry had already showcased his prowess in the Triwizard Tournament, demonstrating his abilities in various magical disciplines.
* His connection to Voldemort: Harry's connection to Voldemort, being a Horcrux, gave him a unique advantage, especially in duels against Voldemort himself.
While other members of the Order, like Minerva McGonagall and Kingsley Shacklebolt, were powerful and skilled, they did not possess the same combination of raw magical power, experience, and unique circumstances that made Harry the most powerful wizard in the Order after Dumbledore's death.