How do you promote a newsletter?

I've been writing a newsletter for almost two years now, and despite hitting the HN's frontpage with a summary of it once [1] I've never had more than a few hundreds subscribers (the existing ones very rarely unsubscribe, so at least it's good).

If that were a simple website, I would probably take the classic SEO way and build up backlinks and keywords, but this one is different as I mostly write about things I've just learnt, so the target audience is "like-minded people" or "generally curious people", not "people who care about X".

I tried applying for a bunch of directories but that didn't help much.

Do you think there is a better way to gain more subscribers and eventually build a dialogue with them?

[1]: https://news.ycombinator.com/item?id=25582269