I think a key reason is HN has (IMO) a well-designed vote-based moderation system. Flamebait tends to get downvoted/flagged pretty quickly, burying it where it belongs.

Contrast that to pure engagement-focused social networks like Facebook or Twitter, which do the opposite: prioritize showing flamebait, because people are engaging with it and therefore it must necessarily be quality content!

HN does not have out-of-control mobs and flamewars because HN has dedicated human moderators who monitor hot conversations and use a variety of tools to de-escalate them.

Voting manages the day-to-day and gives them signal to work with, but ultimately open communities (i.e. that anyone can join) need active moderation to remain stable over the long term.

Reddit works the same yet does not has the same feeling at all except for niche subs maybe.

Agree. Reddit has a unique ability among large social networks to bring up sanity and good discussion. If there is misinformation being being spread I'd expect information disputing it to be in the comments 90%+ of the time. (Except for the niche subreddits as you mentioned)