The vast majority of smartphones don't allow installing another OS. Multiple Android OEMs have been restricting or fully phasing out supporting it. Among devices which do permit it, none have provided the hardware-based security features or driver/firmware update support needed by GrapheneOS beyond Pixels. Our hardware requirements are listed here: https://grapheneos.org/faq#future-devices

GrapheneOS has an official OEM partnership with Motorola Mobility and a subset of their next generation devices will be provided official support for GrapheneOS. They'll be providing us with a more minimal form of hardware support code close to the standard Qualcomm and other vendor code, so it will be cleaner than Pixels. Our partnership with Motorola is non-exclusive so we're free to support other devices with the help of other OEMs interested in meeting our requirements, but no other OEM is working with us yet.

We can't use devices with an end-of-life Linux kernel, no firmware updates, no driver/HAL updates and no support for important hardware-based security features we use. Several devices of a lot of the way towards providing what we need and several next generation Motorola devices will provide it. Other OEMs can do the same.