Because the space is limited to 200 items. As someone who has all jobs leveled and changes Glamour frequently, this could pose an issue in terms of the limited storage. I have 2 retainers still dedicated to Glamour after putting several items in the Dresser. It's not worth it to me to put an $18 cash shop item in the Dresser if I have zero choice in removing it later to free up space. Thus, I only put things in the Dresser that I won't be bothered if it ever comes down to me having to Destroy the item to free up spaces.
Honestly, all cash shop outfits should be Armoire-storable. That would eliminate this issue entirely since you can access Armoire items through the Dresser without using up any of your 200 slots.