What are some reasons why a business could make more profits selling through one retailer than to many retailers?

One of the reasons is the appearance of exclusivity. If you only sell through one retailer, and your product is harder to find, and it is perceived as a luxury product, then you could charge higher prices and make more profit. One retailer doesn't always mean harder to find... some mass retailers will give you extra money in a contract to be the only retailer, and the only way for people to get your product. If it is popular, you could negotiate a higher exclusivity clause. Another reason is that there will be no competition sales-wise. One retailer isn't going to slash prices on your product to outsell the competition. That protects you from the perceived devaluation of your product.