What causes online communities to lose quality discussion as they grow?

I've noticed a pattern where once a subreddit or Lemmy community reaches a certain size, the front page becomes mostly memes, recycled jokes, and lowest common denominator content. Genuine discussion gets pushed out unless the community is extremely strictly moderated for it.

What do you think causes this shift? Is it inevitable with the upvote/downvote system rewarding quick, agreeable content over nuanced takes? Or are there platforms or moderation approaches that successfully scale discussion without turning into an echo chamber?