Plastic bags have been favored by retailers for as long as they have been available. That is mainly because they are convenient to store, cost-effective and easy-to-use. In bulk purchases, plastic bags can be bought at a cent a piece. On the contrary, paper bags usually cost 5 cents or even more, while reusable cloth bags can cost a few dollars each.
There is an apparent overhead cost benefit involved for retailers in purchasing plastic bags as opposed to paper or cloth bags, and this would definitely have a good impact on their profit margins.
Moreover, it is quicker and easier to open and pack with plastic bags. Though a few seconds difference may seem to be nominal, in busy supermarkets, it can help cashiers efficiently keep the lines moving.
Plastic bags do not require much space either when compared to paper bags, both at cashier stations and in storage, while their weight is not much of a concern either. Reusable bags, on the other hand, take a lot of space and are significantly heavier.