Ask HN: 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