The loads on the airframe is a product of the accelleration (g load) and the mass of the airplane; that means, for the same build strength, with a lower design load limit expressed in G, the allowable mass is higher. That means you can put more useful stuff, like freight, people and fuel in the plane, without making it stronger. It's simply not nescessary to build air transport planes for higher loads, since building them is always a struggle between strength and weight - a stronger design is normally heavier, which in turns eats away the benefit. It's a delicate compromise.