How does the solar nebula theory help us understand the location of asteroids?

The "Asteroid Belt" between Mars and Jupiter, once thought to be a destroyed planet, is most probably the result of Jupiter's powerful gravity preventing small bodies from aggregating into decent-sized planets (the biggest asteroid, Ceres, has received the designation of "dwarf planet").

Most of the material which might have gone into planet-making probably fell into Jupiter, with little Mars just getting two smallish chunks which became its moons, Phobos and Deimos. On the other side of Jupiter, Saturn competed for asteroids, and between those two giants the region was pretty well swept up.

There are still some asteroids scattered throughout the Solar System, including a few that are close enough to Earth to concern us.