I haven't poked at the game's memory space for that, but in all honesty the armoire is probably a giant bitfield -- a big series of on/off switches, one for every single item that can potentially be put in the armoire, with the switch toggled 'on' or 'off' depending on if you have the item in there. Which is an awfully nice way to reduce the need for storage when you have 'list of gear X that would be in the glamour dresser' and can condense it down to only a single 1/0 bit for each item (meaning one byte could be used to store 8 items in the armoire).
But I'd imagine every time they add more things to the list of 'what can be put in the armoire', that bitfield probably gets longer for every single account. So if they add 16 new possible items to the armoire, the armoire of every single account gets two bytes longer. And if you add every single mogstation item and seasonal event item and level 1 glamour item (like the "Scion <whatever>'s" outfits) -- all the corsages, all the earrings and whatnot -- I imagine the added size multiplied by the number of players is something they'd be concerned about.
In contrast, I expect the glamour dresser is just a fixed-size array, like 400 little boxes which can each contain a (very large) integer which is the ID of a piece of gear. So you just have 400 boxes, and even if each of those boxes were 8 bytes in size (so you can stick a 64-bit number in each, because FUTURE PROOFING), that'd still be only 3200 bytes per glamour dresser. And no matter how many new pieces of gear you add to the game, the glamour dresser doesn't require changes (and the code working with the glamour dresser doesn't need to change, as a result).
Moreover, from what I understand, the glamour dresser puts the player in a weird state, and to avoid problems, that's why glamour dressers only appear in private instances -- places that no one else could be in -- like the inn rooms and squadron barracks.
So as a programmer, I can understand the headaches they might have in trying to move more things into the armoire, or to rework the glamour system such that the glamour dressers could be put somewhere more convenient.
As a player, I really feel like all the seasonal event stuff, Mogstation stuff... anything that's level 1 glamour items, really... should be in the armoire, and the glamour dresser really ought to be more conveniently accessible.
(I cannot figure out why on earth there's any rational technical reason that glamour plates would only work in hub cities and housing areas, though. That one baffles me.)