I think the difference is that only individual posts get downvoted here.
I (used to) have friends on FB who will unfriend anyone epxressing an opinion that they don't agree with (or "find offensive"). They end up in an echo chamber where they never hear an opinion they don't already agree with, and then get surprised when elections don't turn out the way they wanted. Weirdly, I'm still friends with some of them irl, and we can have civil discussions about the same topics in person, just not online.
Unpopular opinions on HN get downvoted to invisibility, but the author is still able to post other unpopular opinions. They'll probably also get downvoted, but sometimes they get supported.