Far too many short platforms between Liverpool and Crewe to make 6 car operation practical even with DOO. Most can only accommodate 4 cars as far as I know. Then there is the question of demand: I'm not sure 6 cars are even needed on this route.
6 cars would very much be welcomed between Liverpool and Manchester in the peaks, particularly the 0720 off Lime street and the 1622 and 1723 off Piccadilly to Liverpool.