Flat forums have to worry more about staying on topic because all the conversations happening in a thread are thrown together. There's no "flow". Posts responding to each other and posts responding to the OP are all jarbled up.
On reddit you don't worry about conversational tangents. You just fold them.
Notice how I didn't have to quote you for others to follow our discussion? That's because our conversation is nested :) If this were a flat forum, my original post would be at the top of the page, your reply would be in the middle, and this comment would be at the bottom. Multiple other conversations would be in between. And we would have to quote each other to have any chance of following the conversation.
Flat forums are strictly worse than nested forums for in-depth discussion.