Truck owners expect their vehicles to be able to thrive under all sorts of conditions. Trucks are expected to handle heavy loads, bad roads and many more. Manufacturers have made it a priority that they provide the reliability and strength that truck owners expect. Owners have complained that speed is the alternative forgone to meet these standards. Some people believe that trucks could move faster and still be as strong if there is a rear bumper diffuser for more stability.
Diffusers allow the vehicle overcome air resistance; this makes the car travel faster. The diffuser reduces the aerodynamic pullback by increasing the vehicle down force. Air entering from the front and under of the truck is expanded through an expansion chamber. Most cars do not have a smooth underside so the air cannot pass under it quickly; this is where a rear bumper diffuser comes to play. For a rear bumper diffuser for more stability, it must have vertical fins that ensure that the flow of air under the vehicle is directed to the rear. This will keep the airflow gentle. The diffuser must also have an expansion chamber that reduces the air pressure so the air can escape from the back. The expansion chamber is responsible for the increase in down force. Diffusers also have a smooth under body to help the car sail over the air passing underneath it.
Trucks do not need rear diffusers because the body of the vehicle is too far from the ground. There is no need to increase the down force of a truck; you do not expect to be speeding into a tight corner with your utility vehicles. The manufactures of sports cars did not everybody kit for glamour, at least not initially. People that buy salon cars these days drive at breakneck speed; this is why their manufactures add diffusers. Trucks have a high center of gravity so they are still great without diffusers.