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They run on Pixels because that's pretty much the best phone in terms of security and privacy.
You can read here some of the properties that a phone should have in order to be able to run GrapheneOS on it. Here's an excerpt:
In addition to support for installing other operating systems, standard hardware-based security features like the hardware-backed keystores, verified boot, attestation and various hardware-based exploit mitigations need to be available.
Why would an alternative not have backdoors but the pixels, the most private and secure phones, would have them? Makes no sense to me.