One of the underlying reasons is probably that the Fairphone hardware and software is developed/maintained by a Chinese ODM (T2Mobile), so the question is if Fairphone even has the know-how to make such a phone. Heck, they have failed to fix bugs that have been plaguing people for months (e.g. regular dropping of all connections when using IPv6 on certain routers).
Motorola is going to make devices that fulfill the requirements and GrapheneOS is cooperating with them.
Hardware and firmware level protections can't be added by software.
Software features based on hardware security features also can't simply be added without those. Translating code to add a software equivalent to MTE could theoretically be done but would make the code require several times more CPU cycles and several times more memory. You would need far higher performance hardware and a bigger battery for that to be practical. MTE is what GrapheneOS uses rather than that.
GrapheneOS would love to support more devices, that "smearing" is explaining that a company doesn't make secure devices, which is completely correct criticism.
Fairphone for various reasons is not able to or willing to achieve the hardware requirements. It's not easy to create some of the worlds most secure devices.