This wasn't always through - the first web forums were threaded more often than not, and even vbulletin supported a threaded mode as a user preference well into

This wasn't always through - the first web forums were threaded more often than not, and even vbulletin supported a threaded mode as a user preference well into the 00s (though as it was no longer the default, others were not using linked replies, which hurt its usefulness at that point). This is probably related to imitating the way mailing lists worked, as some of these early UIs were like the older style of mailing list presentation with a few more forms attached. The flat forums were considered an innovation for a while because "normal users will never understand this nerdy threading model". Arguably to some extent they were right, as mass market products like youtube, facebook, etc. still limit to one level of replies. The real innovation of the social news sites was the voting and scoring algorithms, which made it manageable by presenting users with the most popular subthreads first, rather than the chronological order the forums had used. And gaining those points had a kind of skinner box effect on keeping users hooked on the sites, which helped their growth too - especially when points used to have a much more prominent display.