They are there because they would be unstable in most other places. Asteroids further out would be disturbed by the gravity of Jupiter or one of the other gas giants. Asteroids closer in would be disturbed by the gravity of Mars, Earth, etc. Once disturbed they would be either flung out of the solar system or they would end up crashing into the sun.

Early in the history of the solar system there were many many more asteroids. It is thought that most of these were flung out of the system, become part of a planet, or fell into the Sun.

It is currently believed that the asteroids in the asteroid belt are rocks which never managed to accumulate into an actual planet.In the early stages of the solar system, before the planets were formed, a disk of dust and gas orbited the sun. This matter gradually aggregated to form the planets we see today. Astronomers believe that the strong gravitational field of Jupiter continuously disturbs the motions of asteroids in the belt, thus preventing them from aggregating and forming a planet. The total mass of all the asteroids is actually less than that of the moon.