Part of the reasoning mentioned is the “service does not want to see the trainer program diluted to support other missions”.
Also of note is that Textron’s Scorpion is specifically mentioned as being excluded in the OA-X program. I can understand that. Both the T-6 and A-29, which are being looked at for OA-X, are already in production, and are proven airframes. While their payload may not be as great as the Scorpion’s, they are tooled up and ready to go.