I didn't downvote, but I would guess it's due to the general idea that if you just approve or disapprove of a post you should simply vote that way instead of expressing it in a comment. Personally while I agree there's a logic to that, I find it a little cold for positive sentiments. I couldn't find it, but I think that's there's a PG or Dang comment to the effect that "I like this" as a comment is explicitly not discouraged on HN, but obviously that doesn't mean everyone agrees.
