The problem is the limited resource of space. One line, multiple cashiers takes up more space than several cashiers, one line. The store needs to add the queue system and have a person standing at the front to make sure the line moves smoothly. Think about that large waiting area Fry's has.

Another point is that 6 people standing on a single line for 2 cashiers seems like a longer wait than 3 people standing on two lines.