Question

explain why bus body builders build luggage compartments under the seats rather than on the roof racks

Answer 1

First, keeping the luggage below, in the boot, keeps the center of gravity of the bus lower, making the bus more stable on the road. Second, luggage on the overhead racks is at risk of flying off the racks (and possibly endangering passengers) if the bus stops short or makes a sharp turn (inertia at work here)