It says they are using a "domestic configured Airbus A321neo"; it probably doesn't have the range to make the trip with cargo. The idea being the train pilots on the NEO so they can fly/teach it on the XLR.
It should be able to carry at least 180 passengers times 80kg = 14400kg, right? And airlines don't like losing money, so perhaps it's something else, like not having the infrastructure/licenses to haul cargo?
