Comment by handoflixue

It's fundamentally a problem of volume - forums work great for a small community of a few dozen people; the larger it gets, the harder it is to follow a thread as a dozen people are all replying to a dozen different sub-topics. But in a smaller group, it ensures everyone can follow those sub-topics and understand the overall state of the conversation.

For a larger community, Reddit-style gives you a lot of quality sorting and makes it easier to handle spam by giving the community some ability to "downvote" unwelcome commentary.

Eventually you hit the point where Reddit-style systems break down because they're large enough to attract karma farmers and/or collapse into an echo chamber. We haven't really figured out anything that scales well at that point beyond the Twitter/blogging style of "follow people whose opinions you respect" (the tradeoff there being that you're now mostly in a broadcast mode, rather than a reasonable back and forth, and the "broadcaster" is liable to be overwhelmed with repetitive comments)