Why does HN still have high-quality comments at scale?

It's not really at scale. Everyone is still at least partially aligned. There's no critical mass of militant single cause people to come in and hijack it.

Otoh, it's as big as it is because it's a benevolent dictatorship, not any kind of democracy. It's heavily moderated with enough (I mean this positively here) citizen policing, in the form of moderation and flagging, that it stays pretty much on mission.

Well, it's rather inaccessible/unappealing to the majority of people who use sites like Reddit, Twitter, Facebook, etc, so that probably has something to do with it.

There's a pretty strong correlation between 'folks willing to use a site that looks like it's from the 80s without mobile app support' and 'folks with meaningful things to say', at least if you take the average front page content on other social media sites as a baseline.

Also because Dang does a really good job with the moderation, whereas most of these sites farm it out to the lowest bidder.

It is specialized.

Focused communities are, in my experience, higher quality than more general ones.

Also what version_five said.