Sepulcra
It's a well known sales trick: get it before it's gone.
Gaia uses it constantly because they think (or they have experienced) that it works.
When they believe they can earn money more by bringing it (temporarily) back, they will. For example: the glowing splicing shard. You can get it for only a few hours. A few days later: you can get it for only a few hours - again! Do you still believe what they're saying? Nope? Neither do I. But it doesn't matter because money is more important than being reliable.
I'm not a fan of limited time offers either. I'm convinced that on lots of items they would have earned more if they wouldn't remove them from the shop. But once you go down a road, it's difficult to turn back.
It very much does make sense and I'm in no way arguing that. It just doesn't make that much sense to me for limited quantity items to be limited time as well.
I believe that a dev once stated in an ATS(?) that the number of limited quantity items were there as a sort of guarantee that the item would make back it's production costs, so taking it out of the Cash Shop before it's sold out seems very counterproductive, especially when one considers how badly some items can age, many becoming less and less popular.
I remember when the Hydrangea Dream first came out, it was rather expensive, but now? Not so much...
This also extends to a number of EIs and REIs as well. Prince of the Sea, Secret Retreat and many more have seen drops in popularity compared to when they were still shiny and new.
So it really just makes more sense to me to leave the limited quantity items in the shop until they sell out in order for the item to make back it's production costs.