HN users don't necessarily do that because they want to. They might do it as a pre-emptive defense mechanism against the brigades of de-facto censors that roam the site.
Moderation via populism is an anti-feature on its face, but Hacker News has the worst possible version of that sort of feature by making downvoted/flagged comments completely hidden unless you are logged in and showdead.
It's a pretty horrendous system if you're interested in good faith and honest debate.