Graphene might be great, but the Google specific hardware that is the only thing it will run on might not be as trustworthy.
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.