Single pack operations are sufficient to pressurise the cabin, and since the packs feed the air conditioning manifold the front and the back of the cabin still receive adequate air. The flight deck still receiving air directly from the L pack for any problems. If both bleeds were off then the C1 and C2 demand pumps would need to be fed from the APU. The APU may not be able to supply both packs and the C1 plus C2 demand pump in a high demand state like after take off when both demand pumps are on for gear retraction..... That would be my guess anyway Kapitanleutnant.