Why Racing Car is Bottom Heavy?

Why racing car is bottom heavy?

Do you know racing cars or motorcycles are not our normal vehicles? Most of their weight is on lower side. 

For a simple example, open the matchbox halfway. Stand it in on a cardboard with its outer cover down and inner drawer up. 

Make a mental note of the angle at which the matchbox topples. 

Once again stand the matchbox. But this time stand it with its drawer down and the cover up. Again tilt the cardboard. This time it requires a larger angle to topple. 

Center of the gravity of a non- uniform body is located somewhere near its heavier part. 

So the center of gravity of the half open matchbox will be more towards the portion containing matchsticks. 

When the center of the gravity is higher, it comes out of support base at a smaller angle. 

But when the center of gravity is lower, it requires greater angle to come out of support base. 

Conductors of double Decker buses allow passengers to upper deck only after the lower deck fully occupies. 

Anybody know why do they do so?