Google phones are pretty much the only ones that lets you relock the bootloader with your own signing keys. OnePlus used to, but not anymore. That means anyone can just flash anything to your phone and there's no way to prevent it, except on Google's phones. So, 30 seconds while you're not looking and there's a potentially a keylogger running as root on your phone.

With that in mind I can see why the authors aren't interested in other devices. To release builds for a device you really need to own that device so you can test it on, maybe several of them. Each phone needs its own custom build and hacks and quirks. That's expensive and time consuming. So you need someone with your particular model to be interested and volunteer in porting, maintaining and releasing builds of GrapheneOS for that phone. And the GrapheneOS guys are unlikely to buy those phones in the first place because it doesn't have the features they want for their OS.

There's probably builds floating around on XDA for GrapheneOS, for people like you that don't need the security but just the privacy features. LineageOS' list of official devices is pretty small but there's unofficial builds for damn near anything on XDA, so it wouldn't surprise me to see some unofficial GrapheneOS builds as well. Once you do have a device and a build setup, working on multiple ROMs at the same time is fairly easy, so I've seen the same developer releasing builds of whatever they can get to build.