Interesting question. Probably because Dumbledore was hunting for the horcruxes, and those had to be destroyed before he took on Voldemort directly.
And so, in their duel in OOTP, we see Dumbledore seeking to capture, not kill, Voldemort. But he still wins, in that Voldemort fails to defeat him in the duel.
(The movie version really does justice to that particular part -- it really gives a sense of two awesome wizarding powers taking each other on.)
Maybe, but it seemed that the knowledge of the Horcruxes was late in coming to Dumbledore - he only suspected in the later years and then sought to interview those who could confirm his suspicions.