Their goal is to hold a standard of security and privacy that can be trusted. They want the brand to be associated with that level of security and privacy. Sticking to Pixel allows them to leave their brand undiluted in this respect.

Thats are improving peoples security overall by only using pixels at this time. Nothing else currently meets the hardware security requirements except pixel devices. To use subpar hardware would set the security back substantially. They are not going to compromise on security for the sake of broad device support.

Also note that Motorola Mobility has stepped up to provide the needed hardware security features. So it will expand to a subset of Motorola devices next year.