For your comment Zombie King, that's not exactly how game operations work (though, I cannot know exactly how NCSofts ioerates ganes, or the operations of Blade and Soul specifically))
Specualation on my side would be that they have limited resources and have a bunch of already predefined scripts and templtes for specific sales which they rotate based on last availability and plans, this can lead to some items appearing in duplicate sales over a small period of time. This isn't problem with the game files itself. It is more likely to be a problem of them looking and choosing which item go into which sale. This may also be a limitation put on/by their Production team by making a concious decision to limit the amount of cosmetics that go into the store for the Western version and instead funneling users who desire cosmetics to other monetisation features like the Trove or Lootboxes.
I don't think further speculation will take us anywhere though, I think the main problem is we lack the amount of rotation in the shop. Despite me playing last in 2018 fully, I have come back to the game to realise 90% of the cosmetics avaialble in the shop then are STILL available, with a very small limited amount removed and replaced with other items.
If they were to do 2 outfits a week (which is something they trialed in 2017, and community perception at the time was veyr positive) or 10-20 a month and rotate them out and replace it would encourage people to look regularly at the shop and encourage them to possibly even spend more.
If you are having trouble understanding what types of outfits players want @Green Storm, I would recommend creating a thread with a list of items your team is considering (that way you don't get mostly requests of Trove items which are probably no-gos for your team) and for players can then pick the ones they want via poll. Top ones go in the first rotation, the rest go in the second rotation, and then during the second rotation you do another poll. This would also guarantee a perentage of sales (not everyone entering the poll, but I would estimate around 15% o f purchases) on those 1st rotation cosmetics due to the availabiliy and interest surrounding them.
Just some food for thought, I will continue to wait for more cosmetics to be added ot the store.