Android

The pixel has a TPM chip (Trusted Protection Module). This is a device that has an internal cryptographic key set that can never be accessed outside of a lab with access to the die, like using red fuming nitric acid to get to the die without damaging the gold bond wires – type/class of access.

The internal key of the device is used to create any additional keys you would like to make. It can also test the validity of any key it has generated and check the hash of any file. Hashing a file is ensuring it is unchanged using a key to ensure no one in the middle has altered or added to the file.

Graphene uses this TPM chip to create a chain of trust with the device. It is essentially like secure boot on a computer. Anyone with control over a key like this is able to make changes to your device.

No mobile devices contain documented hardware for the system on chip or the modem. Both of these are black boxes. Obviously the modem is a big concern. Google/Android is a scheme to make it as easy as possible for hardware manufacturers to build with Android using a kernel that only needs the hardware support binaries added at the last possible minute. There is no source code available for these binary kernel modules. The code is never added to the mainline Linux kernel. This is the mechanism used to deprecate your devices. These are orphaned kernels that are proprietary and cannot be properly maintained and updated because of these kernel modules for the physical hardware. The hardware is different enough that no one can reverse engineer it effectively and transfer that effort to other devices.

You can never completely trust this hardware. We simply do not know what is there and what it can do. However, with a TPM module, we can make it irrelevant and simply discard any interaction or activities we do not understand. We can also verify that none of this hardware is altering or interfering with the OS or files on the device. Any changes will alter the result of the files hash. The inability to access or modify the TPM hidden key and its existence outside of the SoC is the reason Pixels are used and why other devices are insecure.

Their FAQ says kind of:

Many other devices are supported by GrapheneOS at a source level, and it can be built for them without modifications to the existing GrapheneOS source tree. [...] In most cases, substantial work beyond that will be needed to bring the support up to the same standards. For most devices, the hardware and firmware will prevent providing a reasonably secure device, regardless of the work put into device support.

The requirements that GrapheneOS has on the hardware, like relocking the bootloader and hardware level access, should be part of rights to repair / digital markets act imo. They are even considering producing their own hardware in the future.

A basic requirement most devices don't meet is the ability to relock the bootloader. Other than Fairphone, Google Pixel and OnePlus basically no manufacturers allow unlocking and subsequently relocking the bootloader, which makes custom ROMs inherently less secure than stock. This keeps CalyxOS from most devices. LineageOS can't be relocked and thus is able to support way more devices.

Others have pointed out more in-depth security requirements GrapheneOS specifically thinks of as mandatory (they do take security very seriously).

Pretty sure they use some google security features that breaks it on non-pixel devices.

It can be done, but it's a pain in th ass and it varies by vendor, model, and version (and region). Whether it breaks any functionality is dependent on those things although usually not. If it breaks something it's usually like the ability to change volume over bluetooth and stuff like that vs - y'know the screen not working or something.