The unlockable bootloader is only a small part of the reason Graphene only works on Pixels. One of the main reasons is that Google actually maintains drivers and firmware updates for all of their components and releases them monthly (well, usually) unlike other vendors that are often one or more months behind. Many (most?) other phone brands simply don’t provide the security guarantees that Graphene desires.

There are GrapheneOS ports for other devices, and ROMs that take GrapheneOS features, but none that I know of have the security that GrapheneOS has.