Because even here on HN where conversations are, for the most part, fairly great you still have the issue of new content coming up daily and so the conversation has to move on the new topic.
I personally had luck using emails to interact with people but I was wondering if other people had different experiences.
Any forum with open membership will eventually attract people who can't/won't handle nuanced conversation. The only way to completely be safe is with a moderated mailing list.
For long conversations, look for moderated, special interest forums. I think HN is as good as a general purpose forum can be, with its heavy moderation and fairly restricted audience.
Heck, I've had a hard time teasing out the nuance of some of my points when debating with a single friend over phone. For almost two hours. This sort of thing only really works in high trust situations where everyone assumes good faith all the time.