Because this refit is not a standard refit. airlines are free to choose whether they want to have the HUD installed or not. For the China Southern Airlines case, I believe it’s because they’re bound by mandatory government law that requires the A320 to be refitted with the HUD. But for other countries, it depends on the airlines. These aren’t cheap installations.
Another challenge about HUD is the collimation effect. The 787’s HUD while there, it’s not really optimised for a good collimation, the horizon is always off from the actual horizon and they don’t “stick” when you move the camera around as it should.
For now, we can only hope that the FlyByWire team is implementing the HUD in the future. I already request an enhancement. But since this requires it’s own unique 3D model as well as another set of functionality that uses more CPU processing power to simulate, that will take some time.