Because users flagged it.

A better question might be, "why did you flag this post?", although those who did won't likely see your comment.

Edit: Your account is from 2012, are you still learning how this site works?

I do know how the HN works. But I can't see who flagged it. So it's really a genuine question - I don't see anything about the site that is against the guidelines. And I'm trying to understand why whoever flagged it did so.

For posts, flag seems to be a proxy for the missing downvote button, to many users. Sad, but that's the way it goes.