Because The SC tracklist interface is per layer and per player, so the X1800 mixer knows which fader is closed or open.
The Prime4 tracklist interface is for all layers. You choose after selecting on which deck you want to play/load.
Preview would be easily possible, but not with a faderlock or precheck. To harm you for making mistakes, they need to implement a preview with deck choice before preview audio is heard.
Do you understand my reasoning a bit?