Why is it that all of the matter in the inner solar system accreted to form planets, while in between Mars and Jupiter there is still a bunch of stuff floating around?
Conservation of energy is kind of tricky. As shepherd objects pass by the asteroid belt, they either give or take away orbital energy from the asteroids, pushing them into larger or smaller orbits. This is the reason for why these asteroids can collide with terrestrial planets. This resonance also has another effect of destabilizing these orbits so that the would-be accreting material never actually accretes. Each collision serves only to break up large asteroids into smaller ones.