Why do people like to comment here instead of the original post?

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.

There are established accounts here. On an arbitrary link page, if comments are supported, then they might support for anonymous accounts, or you have to have your own account (with registered email/password), or use Disqus/Facebook or other third-party comment system, or login through OpenAuth.

There might be user-preview and edit, or not.

It might have to go through an administrator review before posting.

There might be a way to be get notices about followups. Or not.

And on HN, sometimes I read the comments to see if it's worthwhile to follow the original link. For example, sometimes the link seems like spam-bait, but the comments suggest otherwise.

People commenting here could interact with other people and their comments could be appreciated by others (voting).

In theory the community here should be startup oriented which gives it a unique point of view.

yes, a hot thread here might only generate few comments on the original post.

Irony.