Graphene is not limited to Google hardware, it's just that Google hardware meets their hardware requirements... (https://grapheneos.org/faq#future-devices).

AFAIK (from observing GOS comm channels) verified boot (alternative OSes but even the mechanism itself since some OEMs customize quite a bit), hardware rate limiting and timely security patches (which include modem firmware, preloaders - i.e. hardware) are the main reasons other devices are not supported.