Actually there is a reason. They coded it differently than the rest of the inventory system because simply adding that space to the current inventory system would mean that it would be subject to all the same operations as the normal inventory. Specifically in this case they were looking to maintain the stability of the Duty Finder, because you always have access to your normal inventory by default and if they added these slots on to the normal inventory it would increase the amount is data to be transferred when loading into an instance. They are very leery about increasing this amount and have actively worked to reduce it as much as possible -i.e.: no minions in dungeons, no repairs for other players in dungeons, no item trading in dungeons, etc. (Though that last one also preserves individual server economies by prohibiting trading large amounts of items and gil by RMT across servers through the Instance servers...)