Reddit made it impossible to have long-term discussions like forums do. Posts would just fade into irrelevance after a day or so, whereas with forums new comments would bump the thread.
Lemmy has a sorting option just like forums:
“New Comments: Bumps posts to the top when they receive a new reply analogous to the sorting of traditional forums”
And Lemmy also has the “Active” sorting method, which says:
“Calculates a rank based on the score and time of the latest comment, with decay over time”
so Active seems like a compromise between Hot like Reddit and the way forums do it by New Comments
https://join-lemmy.org/docs/en/users/03-votes-and-ranking.html
It’s probably because the algorithm was tuned for a smaller community. As soon as posts got hundreds of comments it likely made things stay at the top for a very long time. Definitely something that will get tuned over time.