Here's why:
* The Elder Wand's Allegiance: The Elder Wand only changes its allegiance through a defeat. The wand's owner must be defeated and disarmed by the person who wants to become its new master.
* Draco Disarming Dumbledore: While Draco disarmed Dumbledore in the Astronomy Tower, it was not a genuine defeat. Dumbledore willingly allowed Draco to disarm him as part of a plan to ensure Harry's safety.
* Harry Defeats Malfoy: Harry ultimately disarmed Draco, taking the Elder Wand, but Draco never truly possessed it.
* Harry Defeats Voldemort: Harry disarmed Voldemort, who had previously stolen the Elder Wand from Snape. This is the true moment when Harry becomes the master of the Elder Wand.
Therefore, Draco Malfoy never held the true power of the Elder Wand.