The thing with Nostr is that the protocol spec expressly forbids relays from forwarding messages to each other.

What this means is that users trying to reach each other need to shotgun messages to many relays, or congregate around specific ones. User profile data will list the relays they listen on, but this suffers from the problem of sticky defaults and makes client authors the kingmakers.

There's lots of centralization pressures like this that the protocol maintainers don't have a good answer to. They tout the simplicity of the protocol, which is often a virtue, but they overdid it and made the protocol too simple to achieve its goals.