If we look at something like Reddit, back when I used that, if I had a really popular comment I would feel the need to go though every notification, read it, and respond if it was warranted. This could end up being a lot of work. It was also an issue, because Reddit’s notification page was broken (last I used it), so the unread list would break when trying to go to a second page. So to catch them all, one had to note down the number of replies, then count, going page by page, until the comment count viewed matched the noted notification count. This was quite a pain.
Here on HN, if I have a popular comment, I can see that it generated a lot of discussion and tend to scroll through it and read/skim it like I would most other comments, especially once they start nesting further.
The net result of the HN model feels like less stress, as notifications imply some form of obligation. Why would I be getting a notification if it wasn’t important?