This is not an official answer by any means, but I'm guessing it is because the group as a whole is discontinuing Android 11 variants, and I have not seen any work done for Android 12 on OG Pixel(XL) devices. So, if support were to be continued, the mantainer is venturing into potentially uncharted waters.
As for if Android 12 can be ported to OG Pixel(XL) devices, I'm not certain, but considering it is coming up on its fifth year, the entire development community around it may have moved on to later more relevant devices.
In addition to the above, majority of maintainers do this in their spare time, so I would guess if they maintain multiple devices, their priorities are currently with other phones rather than a nearly 5 year old phone.