Large or "big box" retailers wield enough power in the market to dictate policy including "buybacks". This is when an item is not sold, for whatever reason, the manufacturer is required to buy the product back at the retail price, therefore freeing up valuable shelf space for other products. This shifts the risk of sales back to the manufacturer who is then incentified to advertise thier products, and the retailer, to ensure goods move off shelf to consumers and not back to the manufacturer.
This is also where discount warehouses obtain stock. They purchase this "overstock" at huge discounts due to the manufacturer's inability to absorb the cost of re-shipping and storing buyback product.
This lack of risk and fixed costs, allows the big box to expand rapidly, therefore expanding it's influence and also it's ability to dictate price. This is why WalMart is inexpensive and everywhere.