Why are my iPhone 7 and 7+ purchased directly from Verizon not compatible with Xfinity Mobile?

I have two CDMA phones (iPhone 7 and 7+) purchased from Verizon that are currently active, but when I try to port them over to Xfinity Mobile when I type in my IMEI, Xfinity says the phones are not compatible. Both iPhones are unlocked.

I tried to call Xfinity to ask them if they could explain why they are not compatible since Xfinity Mobile uses Verizon's LTE network, and they said the IMEIs are listed on their backend as incompatible and the only option I have is to purchase a new phone.

Xfinity/Comcast says they don't have a way to update the list of compatible IMEIs on their backend. Does anyone know if this list of compatible IMEI devices is maintained by Apple? Is there a way to request the lMEI list be updated because it makes no sense to me why both iPhones work on Verizon, but not on Xfinity Mobile, which is essentially using the same Verizon backbone. Thanks.