Why does a heavily loaded vehicle need a longer distance to stop?

When a vehicle is heavily loaded, it puts more weight and pressure on the braking system. Spring brakes, which are a type of parking brake, typically do not engage on all axles of a vehicle. Instead, they are commonly designed to engage only on specific axles, such as the rear axles.
During braking, the weight transfer and increased pressure on the braking system in a heavily loaded vehicle make it more challenging to bring the vehicle to a stop. The limited engagement of spring brakes on specific axles may not provide enough braking force to stop the vehicle efficiently.