To clarify, there are three separate versions of Final Fantasy XIV running (i.e. with entirely different account and subscription systems, handled by different companies).
- The 'global' version we here all play with the NA/EU/JP datacenters, run by SquareEnix. This is the main version of the game, run by the developers themselves.
- The version of the game in China, run by Shengqu Games. Shengqu licenses the game code and content from SquareEnix to run their own copy of it with their own account system and servers. SquareEnix has no involvement in the running of the Chinese version, but as the core codebase updates with new features and content, those are provided to Shengqu.
- The version of the game in South Korea, run by Actoz Games. Like Shengqu, Actoz just licenses the game code and content from SquareEnix to run their own copy of it.
My understanding is that the China-exclusive items are added to the game not by SquareEnix but by Shengqu; the Korea-specific ones are similarly created by Actoz. As a result, the content isn't SquareEnix's to use unless Shengqu or Actoz gives it back to them. (This would also presumably be why China and Korea have separate bits of exclusive content, as Actoz would also have to license China-exclusive things from Shengqu and vice-versa.)
To give an analogy, it's like if Company A makes a car, and Company B buys the car from Company A and then makes aftermarket modifications to it and then sells the modified/upgraded car to the general public. Company A can't say "you have to give us those modifications to put into the cars ourselves" (unless they wrote some sort of draconian terminology into the contract that anything you do to Company A's cars becomes the property of Company A). If Company A wants Company B's modifications, they're now going to have to go to Company B and ask to buy or license those modifications back in return.
Demonstrably, SquareEnix is willing to go to the table to negotiate to get to use some of the content Shengqu or Actoz make for their respective cash shops when people demonstrate there's a demand for it in the global version; we've gotten other exclusives made global before.
Why this specific set hasn't been pulled over? Presumably only SquareEnix (and maybe Actoz) know the reason. Maybe Actoz got the content from someone else and they don't have the rights to sell it on to SquareEnix. Maybe they're asking too much for the content and SquareEnix doesn't want to have to pay for it. Maybe SquareEnix looked at the clipping on that dress and went "Yikes, no."
Either way, it's not like SquareEnix makes the content and then hoards it to only be available in one region.