The reason for it is simple based on design features of the NG.
The cabin is controlled by 2 packs, both feeding into the mix manifold.
The flight deck compartment is directly taken from the Left pack only, no air from mix manifold. Thus Left pack must always be used in this case.
Note that this defect and use is not the case for a Pack being inop, we are talking PRSOV here.
Furthermore, the details are in the rest of omitted text. Namely that:
- "Airplane is not operated into known or forecast icing conditions"
- "Valve is secured closed before eying start"
- From a maintenance point of view, the engine bleed valve is locked CLOSED.
- "Operating two air conditioning packs with one engine bleed source is not recommended"
