It's all about how stiff the belt is. Most casual belts are made to be very soft for comfort reasons. The problem is that also allows the gun to sag due to the weight and forces you to wear the belt tighter in order to keep the gun in place. A stiffer belt will allow you to wear it a bit looser and still keep the gun firmly in place.
The leather is treated a bit differently and most are thicker or have a polymer stiffener inside.