My guess would be that it would take too much extra design work. Making the current upper stage reusable is necessary for the HLS contract anyways, and they've

My guess would be that it would take too much extra design work. Making the current upper stage reusable is necessary for the HLS contract anyways, and they've got a lot of work to do already on refining the heat shield & fin protection, enabling landing, and working on propellant transfer.