The only reason is one of wear and tear on the ground unit. The packs are perfectly happy to run using the GTSU in ECS mode. One pack run in such a manner is actually more efficient in cooling the cabin than a refrigerated air truck used over the same period of time.
The only related area of concern is that you must never supply pneumatics from ground and APU sources at the same time, unless you want to go AOG for the resulting duct damage.
