the site/social structure. partly it's that there's so much more content now, but also everyone would see the same posts on the frontpages of /b/ or sa or ytmnd or r/all and they'd often stick around for a day or more with further edits in response. on an algorithmically driven platform, there aren't any posts that everyone who logs in on a given day will see. the more diluted their exposure, the less they're able to spread and maintain a second life outside of their primary point of exposure.