Comments with so little value that they get downvoted already cost the commenter karma.
Most posts that have not reached the front page yet do not reach the front page at all. Comments on pages that do not reach the front page have much less opportunity to receive upvotes. This proposal risks discouraging comments on submissions that have not reached the front page; since some submissions only reach the front page because of the comments, this would harm HN.
You also don't want to penalize comments on articles that have dropped off the front page. They're useful to people who get to an article through Google. These readers are much less likely to be logged in and thus, much less likely to upvote.
Most posts that have not reached the front page yet do not reach the front page at all. Comments on pages that do not reach the front page have much less opportunity to receive upvotes. This proposal risks discouraging comments on submissions that have not reached the front page; since some submissions only reach the front page because of the comments, this would harm HN.