Why did Chevy us leading/ trailing brakes on the.

Because you cont have a duo servo brake system on your truck. The light duty trucks don't need it. It'a a basic type of syteme used for many years on many different vehicles.

They do but you have a light duty one so it's not needed
they normally put it on heavier duty trucks that year or they went to disc brakes in later years as well as abs, you may find some two wheel abs on some chevy trucks that year. Ford as well as dodge probably have it that year on light duty trucks.