There are more devices supporting this than there used to be though. https://grapheneos.org/#device-support explains that it's going to support other devices. It doesn't support the Pixel 3a and Pixel 3a XL yet either. Supporting each device is a lot of work, and other devices will need to be carefully chosen. It would be harmful to make bad choices about device support and encourage people to buy insecure devices with too many issues that can't be fixed with another OS.