So Graphene is actually not limited to the developed/western world. As for not supporting other devices, I believe the reason could be the team size and the fact that the fragmented Android world is known for unique shenanigans of every OEM. Besides Google's update/upgrade cycle is another reason it is an appropriate choice.
You can have the best alarm system in the word, if you leave the back door open and anyone can just walk in from the street.
You could just have some disclaimer on the grapheneOS site that says something like "Works best with pixel phones" or have some long password requirement on non-pixel phones
Yeah, that's completely how security works...