That's a fair question. I think upvoting tends to provide positive feedback and encourages discussion. Downvoting (particularly the uncommented variety) tends to provide negative feedback and discourage discussion.

The point of HN is to encourage discussion ergo, upvoting is, in general, in line with the point of the site.

I agree that upvoting can also create an echo chamber, but since it doesn't outright suppress contradictory voices, the effect is much less pronounced.