Not a 737 guy either. but I am guessing the same. ie that left pack gives priority to the cockpit. I will take another stab in the dark and say that by having the left pack operating in a single pack ops also ensures rearward flow of air in the event of smoke in the cabin. 3rd stab in the dark.. boeing figures that in the event of a bleed fault in flight, which could be associated with smoke. better leave the isol valve closed to prevent cross contamination of the bleed system.