When there's a major ongoing story, HN gets flooded with submissions about it, the majority of which just hammer on the same nail and add little that is new or interesting. During such waves, moderators downweight follow-up submissions unless they contain significant new information [1]. If we didn't, the front page would consist of stories about this one theme. Worse, that would accelerate the problem, since once they saw this, people would start submitting even more stories about that one theme. (There's a CS term for this situation—I want to say thundering herd, but is that accurate?)

We hit on this strategy after the Great Snowden Storm of 2013 [2], when many users justly complained that HN was overrun with stories that, though not strictly dupes, were filling the front page with dupiness. It has since proven itself through many epidemics, no pun intended.

You can also derive it from first principles. The thing we're optimizing for is intellectual curiosity [3]. Curiosity withers under repetition [4], so the most important thing for HN to avoid is being predictable. To a first approximation, you can say that HN is ok to the degree that it's unpredictable. But since a page of random numbers would do that even better, it's more precise to say that it needs the right balance of familiarity and unpredictability.

[1] https://hn.algolia.com/?dateRange=all&page=0&prefix=false&qu...

[2] https://hn.algolia.com/?dateRange=all&page=0&prefix=false&qu...

[3] https://hn.algolia.com/?dateRange=all&page=0&prefix=true&que...

[4] https://hn.algolia.com/?dateRange=all&page=0&prefix=true&que... and https://hn.algolia.com/?dateRange=all&page=0&prefix=true&que...