What is the reason there is a belt of asteroids between Mars and Jupiter?

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.

Its due to Jupiter's huge huge gravitational pull