We don't know. It could be material that just couldn't condense
into a planet in that region.
(In fact, the way the planets are arranged, it turns out that the
average distance from the sun to the center of the asteroid belt
is exactly where we would expect to find a planet.)
If the asteroids are actually material that failed to condense into a
planet, then the huge gravity of Jupiter might have been responsible
for keeping the material disturbed.
