The threaded model simply scales better. A Reddit thread (on the old interface, before they ruined it with the redesign) with 5,000 comments is a huge pain to navigate, but it's possible. A flat forum thread with 5,000 comments is utterly and completely hopeless; lasciate ogne speranza, voi ch'intrate. And yes, I was cutting my teeth on flat forums for years before Reddit even existed (all thankfully deleted now, to the relief of my teenage self). The ability to isolate and instantly collapse irrelevant subthreads and understand the full context of a specific reply is what makes it easier to scale threaded discussions.

If you have a small, boutique forum with less than ten active chatters at any given time, then go flat, sure. You don't need to scale when N is small.