Not sure if I'd use GrapheneOS even if it was available on other devices though. Really not a fan of the hostile attitude towards rooting when it's needed for basic functionality like backups — actual, reliable backups for every app, unlike those provided by the built-in solution.
Compare that to other custom ROMs, where you typically depend on volunteers maintaining the various devices. Sometimes people decide to step down, and suddenly find your device unsupported. This happened to me with LineageOS/CyanogenMod.
My understanding is also that the OEM ROMs of Pixel devices are closer to AOSP than those of other vendors like Samsung. This simplifies the maintenance of the ROMs, and enables the project to develop meaningful features instead.