I am new to HN and this is my understanding based on modest time I have spent here. Keeping discussions to HN helps keep them centralized. Secondly, there's a community here. It's like discussing/debating a topic with your friends. People know each other and have rapport within the community. There's a trust factor when you are among friends and you know there's credibility in the advice that's given out.

Commenting directly on posts is somewhat like walking into another group. You are not part of that group. There's no trust factor.