It may be worth noting that GrapheneOS in most cases to date are not the initiators for conversations around extra device support. They do not control which mobile divisions and engineering teams can come to them and back genuine interest with the resources needed to reach an acceptable privacy/security standard for support.

The question is really why are Motorola the only ones that have gone that extra mile so far and what does it say about the rest of the Android OEMs (including Fairphone, which unlike most is actually a younger project than GrapheneOS).

I don't see how the former has anything to do with the latter.