I created a remote team-focused job board. How do I get companies to list?

I created https://goodremotejobs.com. I spent significant time creating a self-onboarding flow for companies to onboard themselves.

I've emailed and Twitter DMed about 30 companies (that are a good match for the site). So far, I haven't received even one reply (much less a signup).

I've made listing 100% free while we're in the onboarding phase.

Other than continuing to reach out to companies (and following up with those I've already reached out to, though in a manner that's careful to not be annoying), I'm out of ideas.

What am I doing wrong? What else should I try?