Ask HN: Why Did the Original Slack Post Fall Off the Front Page?

https://news.ycombinator.com/item?id=30428142 was there originally, gained steam

https://news.ycombinator.com/item?id=30428788 then seemed to replace it

And the second post is only 2nd position, though it's clearly hugely impactful

What's going on that the algorithm downshifts the original and 2nd places the replacement?