Main Belt Asteroids
The formation of the asteroid belt is a result of Jupiter's quick formation and gravitational domination in the early Solar System. The large mass of Jupiter interacted with small bodies in its vicinity gravitationally. Soon objects near Jupiter acquired larger than normal speeds. This, in turn, caused objects to break each other apart during collisions instead of the normal planet building process which requires objects to stick together in order to become a bigger object. Instead of an accretional process there was erosion. Thus the objects in the area of the asteroid belt were not able to coalesce into a planet.