According to the FAQ (warning: very wordy):

The goal of the project is not to slightly improve some aspects of insecure devices and supporting a broad set of devices would be directly counter to the values of the project.

The expectation is for people to buy a secure device meeting our requirements to run GrapheneOS. Broad device support would imply mainly supporting very badly secured devices unable to support our features. It would also take a substantial amount of resources away from our work on privacy and security, especially since a lot of it is closely tied to the hardware such as the USB-C port control and fixing or working around memory corruption bugs uncovered by our features.