Because when the question asked is, "Why is this story on HN?", there's no way to tell whether you're asking "Why is this non-technical, non-belonging story her

Because when the question asked is, "Why is this story on HN?", there's no way to tell whether you're asking "Why is this non-technical, non-belonging story here?", which is highly disfavored, and "What precisely interested the submitter about this story?", which is, well, not terribly important.