Question

Why might stores choose to list $5 items for "two for $10"?

Answer 1

To encourage customers to buy more than one item :)

Answer 2

To encourage customers to buy more than one item
If an item gets sold out, the owner gets more money in his pocket. It's like a cost-plus system, but in this case, the plus comes out to play when an item is all sold out, so the owner has to put in new stock in place of that item.