That's both a matter of design and scale. Forums larger than ~10^2 -- 10^3 participants aren't really discussions so much as a compilation of hit-and-run pieces. For very large subs (10^5 -- 10^6), discussion is effectively over within a day, if not hours.

A strong contrast are the now-defunct Google+ and (at certain points in its evolution) Ello, and the not-quite-dead-yet Diaspora*, all of which had or have a "notifications" pane in which recent discussions are presented in full, and to which all or most prior participants (so long as they've not muted the discussion) see not only direct responses but new comments. I've seen specific conversations continued for days, weeks, months, and even years, productively, and it's a really good way to noodle at an idea (particularly with a good post moderator) over time.

There are other factors about that which also contribute. All three of these platforms are post-and-comment style (HN can be somewhat like this), where the post author is the moderator of that post (HN is NOT like this). I consider the post author a "host", and think of this as a "salon-style" network. Barring blocking actions (which can have a pretty profound impact, TBH), everyone is mostly seeing the same discussion.[1] All three platforms also showed comments to posts in strict, flat, chronological order. As a long-time user of threaded interfaces (tin, mutt, /., Reddit, HN, etc.) this grated strongly on me for quite a while, but I eventually came to see the model as useful and with merits. It's an option and a tool in the box, though I'll still say it's not appropriate in all cases.

I'll also note that G+, Ello, and Diaspora* all have relatively small limits on discussion, with a post permitting up to 500 comments, so while the platform sizes could be large (G+ claimed over 4 billion "accounts", though highly actives were ~10 million or so), individual discussions tended to be fairly small. I'll also note that the salon-style engagement was fairly rare, many people (and institutional/organisational profiles) treated the platforms as post-only or broadcast, and my experience may well be niche. That said, there were some awesome discussions on all three of these which simply don't seem to gell elsewhere: Reddit, HN, and the Fediverse come particularly to mind.

Specific to HN: it's ... close. Discussions are threaded and moderated (both by mods and members, though by different mechanisms). There's no notification of follow-ups and it's hard to tell when a conversation's really died, or if it's still active. Participation quality is ... mixed, but there are some occasional gems. I do regret that there aren't better tools to surface really-high-quality comments or threads, though there've been some manual processes in the past ("best of" lists). That effort's largely been dropped, for understandable reasons of time and scale.

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Notes:

1. Some day I really should write up my thoughts on blocking. Effectively, it seems to create a geography or topography of the social network, which combined with follows and inter-participant links gives paths of greater and lesser propagation. Blocking by highly-linked nodes in particular can be devastating to reach, particularly in a simple chronological timeline. I'm a fan of the mental-hygiene aspects of blocks,[2] but have to admit that being on the receiving end can occasionally hurt. But beyond that, the effects on how following and blocking shape the overall network and activity dynamics is simply fascinating to me.

2. <https://toot.cat/@dredmorbius/104371585950783019>