The google pixel line were the only smartphone you could buy new on a brick and mortar shop on monday almost anywhere in the world and have it running on a custom rom on tuesday and graphene was leading the way in pushing privacy and security at its maximum while other rom maker are making a lot more concessions.
LineageOS still is king to save old devices but grapheneos definitely is a major player because some people like me did chose second hand google pixel with the sole purpose of being able to run graphene.
Ironically, running a google smartphone was the best way to live a life without sending data and rely on google services.