Eu não acho que haja qualquer resposta canônica, mas eu sugiro que você tenha uma série de insights mais profundos sobre o personagem de Dumbledore nas Relíquias da Morte.
Destes, eu suporia que inicialmente Dumbledore escolheu não fazê-lo. Eu poderia dizer que esta é uma das vezes em que Dumbledore e Gandalf (Senhor dos Anéis) são muito semelhantes em natureza.
'But I have so little of any of these things! You are wise and powerful. Will you not take the Ring?'
'No!' cried Gandalf, springing to his feet. 'With that power I should have power too great and terrible. And over me the Ring would gain a power still greater and more deadly.' His eyes flashed and his face was lit as by a fire within. 'Do not tempt me! For I do not wish to become like the Dark Lord himself. Yet the way of the Ring to my heart is by pity, pity for weakness and the desire of strength to do good. Do not tempt me! I dare not take it, not even to keep it safe, unused. The wish to wield it would be too great, for my strength. I shall have such need of it. Great perils lie before me.'
Dumbledore escolheu passar a vida ensinando, protegendo e capacitando os outros. Ele parece ter claramente feito a escolha de deixar de lado o "poder" após os eventos em torno da morte de sua irmã, Ariana. Caçar Voldemort teria sido um ato aberto de poder, algo que ele passou toda a sua vida evitando.
“Years passed. There were rumors about him. They said he had procured a wand of immense power. I, meanwhile, was offered the post of Minister of Magic, not once, but several times. Naturally, I refused. I had learned that I was not to be trusted with power.”
“But you’d have been better, much better, than Fudge or Scrimgeour!” burst out Harry.
“Would I?” asked Dumbledore heavily. “I am not so sure. I had proven, as a very young man, that power was my weakness and my temptation. It is a curious thing, Harry, but perhaps those who are best suited to power are those who have never sought it. Those who, like you, have leadership thrust upon them, and take up the mantle because they must, and find to their own surprise that they wear it well.
"I was safer at Hogwarts. I think I was a good teacher—"
Uma vez que a profecia de Sybill Trelawney foi feita, também ficaria claro que não era o lugar de Dumbledore derrotar Voldemort.
Quando se tratava de Grindelwald, isso era um assunto completamente diferente. Dumbledore sentiu claramente que tinha uma mão direta na criação do problema e, como tal, provavelmente se sentiu obrigado a agir.
“Well, Grindelwald fled, as anyone but I could have predicted. He vanished, with his plans for seizing power, and his schemes for Muggle torture, and his dreams of the Deathly Hallows, dreams in which I had encouraged him and helped him.