Why is does XM sell Samsung A51, but won't allow my unlocked A51? Updated info

It seems like Xfinity is being dishonest here. They sell the A51, but won't allow an unlocked A51 phone that you can get for cheaper. I mistakenly already bought an A51 and now I can't switch to Xfinity Mobile. Verizon accepts the phone and another provider that uses Verizon's network, just like Xfinity does, accepts the phone. Xfinity technical support gives some lame excuse that the chipset it not compatible with the network. I would like to know the real reason why they won't accept my A51.

UPDATE: After speaking and chatting with several reps, they do the same thing that I can do and enter the IMEI number. It is difficult to get anyone to go the extra mile and tell me exactly why they are saying it won't work. There is nothing different with my phone and the one they sell on their website. I finally got a supervisor to at least say that their system won't allow them to sign me up because it won't accept the IMEA. Not because it is not technilogically incompatible. Reps are stuck until this technical issue on their side is remedied. He sent a request to have this BYOD enabled for the A51. I have my doubts as to if this will do any good. 

It is a shame that Xfinity is putting this block in the way of lower income individuals who are just trying to save money, especially in these times. The A51 (all A series phones) is a low to mid tier Samsung phone that appeals to those of us that are trying to save money. But Xfinity is forcing those individuals to pay more for the same phone that they sell. They want to lock you in to their service, because you can't even take their phone elsewhere until you pay it off. (they won't unlock their phone if you still owe money on it.) 

So, the REAL REASON Xfinity Mobile will not allow my unlocked A51 is not technology, it is taking advantage of low to mid income individuals. It is not cost effective to allow me on their network unless i overpay them for their phone or buy a super expensive phone.  Someone from Xfinity needs to turn the switch to allow the BYOD A51 phone and support lower income individuals.