GrapheneOS, in my opinion, can only exist the way they do because of their exclusive support for Pixel devices. It allows them to provide a uniquely high quality ROM with proper support guarantees.
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.
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.