A colleague of mine asked the same question to Airbus
When an aircraft is dispatched with one pack inoperative, Airbus takes into account a failure in flight of the remaining pack and a descent to FL100.
The FL limitation provided in the MMEL ensures that:
The cabin altitude will never be above 15,000 ft during the descent, and
The oxygen masks will not drop during the descent.
Hence the FL limit.
Hope this helps