A couple of pretty rational reasons.

Threaded-forums had a format, which I would argue is specifically designed around their fundamental premise: asynchronous small group single-threaded discussion(even if they have a tree-based format of categorisation to help get to the specific thread/topic).

That works out great for a small,tightly-knit community, and one where you recognise or want to recognise who it is you're actually talking to and are genuinely interested in a long-form discussion. But that doesn't scale well, for several reasons:

1) Humans can only keep track of a small number of identities.

2) Most people aren't genuinely interested in participating in discussion (that is to say, amongst the general cateogry of humans, and by discussion I mean a back and forth sharing of ideas/information), let alone "long-form" discussion over a significant period of time.

3) On forums you're consistently stuck having to follow the path of a given thread, whether the path it takes interests you or not or veers off on a tangent.

4) The typical format of the forum was weighted towards a classification-drilling-down-search to find specific topics on which one specifically interested, rather than a 'bubbling up' of statistical popularity as the foremost mechanism.

Now there are exceptions to all those and forums have differed over time, but bear with me and give me the benefit of the doubt.

Now take something like reddit or hacker news.

Aside from a very high-level categorisation up front (individual reddit communities/the fact that you're the kind of person who actually visits hacker news), everything is just kinda thrown in together and sorted by some measure of recency+popularity: maximising exposure to the lowest common denomintor conditional upon people already signalling they're interested. And the time-weighting is actually just a conditioned way of always giving the impression of freshness and encouraging the desire to check back regularly.

The account names/identities of who you're 'talking' to are now almost irrelevant, because no one can actually keep track of identities across such a large community: with the exception that proves the rule amongst large-scale communities: the celebrity. A handful of well-known accounts that it is partly assumed the general culture will be familiar with. But other than that, its largely effectively anonymous.

The upvote mechanism biases automatically towards popularity, maximising exposure to potential audience. And the tree-nature of the comment threads allows individuals to quickly go to the ones that interests them...but...

By being hostile to depth (it is a pain on both reddit and hn to read anything more than a cursory N steps deep), and allowing the ignoring of individual branches of non-interest, the format again optimises for short-term, shallow, popularity-driven behaviours: perfect for optimisation towards mass adoption.

And lastly, we need to be honest: 90% of people aren't here for discussion (again, the format itself is actually actively hostile to long-form in-depth long-time discussion). They're here to read new content (roughly 24 hour time-based decay of the front page), popular content (upvote-mechanisms, avoiding threads and trees that don't interest them), and when they do post its not because they want to engage with an actual person (they cannot follow that many usernames relative to the size of the community and the format cuts off being practical at a very shallow N deep message length), but like the comment section of the newspaper, its because they have a psychological need to state their opinion/express their worldview/show their knowledge.

Facebook and twitter also have these properties to varying degrees, some of them more explicitly targeting these properties or trying to tailor them to the individual user's demand/desires. Facebook tries to do this literally down to the person's personal detail level and explicitly maximising eyeball time, whereas the likes of reddit achieve this through the channels (users self-select into the interesting communities) and then the upvote mechanism drives exposure within-channels/across-site.

In short, the medium of news-aggregators is specifically suitable for high-scale popularity. Constant quick turnover to deliver returning eyeballs, allows users to filter easily to their needs, low barrier to entry, explicitly tied to popularity to maximize audience, explicitly pushing overly-specialised/alienating conversation off the front page by limiting deep/long-time period discussions and pushing those discussions off-site.