> What causes very active discussions like this to drop off the front page so quickly?

Supposedly posts with very high comments/upvote ratio are automatically classified as toxic and downranked.

That combined with random users flagging it, presumably.

In any case, seems more algorithmic than editorial (which is not to say that the latter never occurs around here in general)

Actually fascinating to really think of it as the inverse of what most social media platforms do these days, which is the opposite.

HNs is a fairly typical "lock threads that degrade to flamewars" strategy that i first encountered more than 20 years ago.