1. We just had a week where more quests were done than usual. This item was likely asked for a lot.
2. The value of this item had gone below what it sold for in main shops. It was removed from the restocking system for a week until it's able to regain value (in this case, obviously, it's become extinct)

Basically before this was the kind of item that sat in its shop all day and no one bothered to purchase to resell because it had lost value, so there were very few of them to actually sell one the time came around.