I think the main thing is that Pixels are one of the few mainstream phones that (for now) allow you to install a custom ROM and relock the bootloader. It all comes down to hardware attestation:
https://grapheneos.org/articles/attestation-compatibility-guide
The world GrapheneOS wants to live in is one where they deploy a ROM which is verified to be secure and to not provide root to the users so that app developers can be confident that users aren't spoofing their location in Pokemon go or taking screenshots of Snapchats or whatever they're worried about. The compromise is that even without root GrapheneOS does return much more control to the user via sandboxing etc. We don't live in this world though; currently any device attestation is inexorably linked to Google.
Notably GrapheneOS isn't advocating for the attitude that "this is the user's device, they should have root and be able to do whatever the fuck they want", they're merely saying that because they've gone to all this work to make their ROM secure it should be considered as such by app developers.
