They did. And for whatever reason, the merchandising manager decided to not stock it this time. They probably buy these candies based on the season. Also, I don't think Reese's chocolates due to well when the weather gets warmer. Do you want a bag of melted chocolates?
There is probably some formula they use to calculate price. None of it is personal.
Let me put it this way. You want to sell 1,000 items. But your store only has room to sell 200 items. There is physically, no place to put the other stuff you want to sell. So what do you do? Just keep selling the same 200 items and never change? Or do you rotate your items so that all 1,000 items can be sold, but just at different times? Think about that.
No marketing. Costco can put Butterfingers on the shelves, but if Koreans don't know what the hell it is then most likely they will not just buy a pack of 50 on a whim. If Nestle contacted Costco and said "We want to sell Butterfingers in your Stores" then Costco/Nestle would negotiate to make that happen. Costco isn't going to go out of the way to sell that niche product.
You have to understand that profits will drive everything in Costco. Why is Suji's cooked chicken breasts sold there and not XYZ Famous US Brand? Probably because Suji's went to Costco and gave them favorable terms to sell THEIR product there. Costco is not "Take everything in the grocery store and sell it in bulk." Costco IS "What items will make us the most money. Oh, these chicken breasts give us a very good margin, and also the company will cover the costs of sampling/marketing inside the store."
When you can understand that, then you can see why business do what they do. Why they sell what they sell.