People prefer cheaper stuff and as know dollar store is the cheapest and it has some pretty useful things just for a dollar.
Well, if you mean why are they out of stock on some random items from time to time, there could be a number of reasons. First I believe they get new stock in once a week. I've also heard headquarters decides what to stock. It's possible that in order to sell items for a buck they may purchase items by the lot, such as clearance, or liquidation items. Or so it seems in at least some cases. But perhaps a store manager may be able to provide a better answer as to how they purchase items.
Try asking their management. I'm not an employee for this company.
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