From the little I know, discourse will probably never offer any official way to allow new comments outside of the main site or emailing. But since it’s open source, it will most likely get a plugin for that, eventually (if it doesn’t have one already).
And the reason is to purposely reduce sporadic engagement. We want real engagement here, and that’s one of the strong points about discourse. 90% drop off isn’t a bad thing if more than 50% of those were useless “+1” comments.