My thought is it's because how HN was designed. It doesn't display images and videos, not even for the ads. Also the reply notification is minimal. When you got a reply, HN doesn't use a great effort to notify you. When you don't notice reply that much, you wouldn't reply to the troll that much. Replying to the troll doesn't yield any good outcome for you because troll just want to troll and not intended for a good argument.

In old Japanese 2ch forum, there used to be a saying "Ignore trolls" and if a person behaving bad, they were said, "Be ROM(Read-Only-Member) half an year." It was a wise word.

Also, the number of users is quite small compared to... say, Reddit.