A model of early solar system formation (and there is evidence supporting such) describes that metal, such as Nickel-iron, rock, and ice condensed out from the accretion disk created as our solar system formed. The metals condensed out first (this is why many of the asteroids are Nickel-iron) Followed by rocky material and ice. These tiny particles then collided creating small boulders and asteroids.
Once these small asteroids and boulders have enough mass, gravity becomes the driving force. Thusly the planets and moons are formed. However, since Jupiter is so large and the total mass of the asteroid belt is so tiny, the material forming the asteroid belt never was "allowed" to form a small planet or moon because of the gravitational perturbations from Jupiter. Remember the asteroid belt has less mass than 1 tenth of our moon.
Finally the solar wind from the newly formed star (our sun) would blow all of the remaining gas into interstellar space leaving us with the planets, moons, comets, asteroids, etc. circling our little star.
Note: This is a really simplified version. There is much (volumes of data) I did not include.
Thx. Did not know that the mass of the asteroid belt was so low. I guess my theory is gone with the wind.