Why is harder to stop a truck than a car?

A truck, except possibly a small pickup truck, will have more mass (weight) than typical cars. This means that for a given velocity, it has more momentum (kinetic energy) that will have to be opposed or removed in order to reduce its speed. The braking systems will have to dissipate more energy to stop the vehicle, and even with larger brakes, they will have to be applied over more time and the truck will travel more distance before it stops. A large truck can also do much more damage if it hits another vehicle or obstruction.