Elsewhere in the great they said they can't support the customer in the right way on RCS. I can't think of any technical reason for right vs wrong support, but I can think of deception as a reason (gaining trust through using a closed platform).

SMS/RCS is better for some use cases (e.g. transactional messaging, promotions, or order updates) while iMessage is better for others (e.g. customer service). iMessage is better for these use cases because it feels more natural to the users texting the number