I also don't get why nobody has written something to provide this. Or, at least, if it has been written, I couldn't find it (perhaps my Google mojo just isn't high enough).
http://api.ihackernews.com/ gets part of the way there, but it's an API, not an RSS feed, and it's broken and has been for quite a while.
So I wrote a script (gist at the link above) to generate such a feed.
You can see it in action at http://jik3.kamens.brookline.ma.us/cgi-bin/hn-comment-feed.c... (which I will have to remove if there is so much interest in this Show HN that people clicking on the link overwhelms my tiny little VPS).