Ask HN: Why don't we have Direct Messages feature on HN?

Feature creep? HN is a news aggregator not a social network; if you want to contact someone and they haven't posted contact information in their profile then they don't want to be contacted.

And if they do want to be contacted, there are better tools than a lossy web-based email-style replacement shim, which is what most other sites end up having.

Are there concrete ways in which direct messaging would make Hacker News better?

HN is not a social network. If you want people to contact you, post your info.