In Dumbledore's commentary on The Tale of The Three Brothers, in The Tales of Beedle the Bard, he تبصرے that the Elder Wand, while being immensely powerful, has never been "unbeatable", as it has been beaten hundreds of times in the constant changing hands of its owners. For example, Grindelwald, though he mastered the Wand, was still defeated سے طرف کی Dumbledore.
سے طرف کی the same token, the Elder Wand does not ensure the victory of its master in a duel, nor render him یا her invincible; Dumbledore mastered the Wand سے طرف کی defeating Grindelwald, but was still unable to decisively defeat Lord Voldemort in their duel in the Ministry of Magic.
Because he was battling Voldemort and trying to prottect Harry at the same time. Didn't آپ notice that every time Harry got up and stood اگلے to Dumbledore, he shoved him back so he was safe?? If he didn't have to worry about Harry, and Voldemort didn't have horcruxes, than Dumbledore would have won.