Why are there asteroid belts?

Answer: Jupiter prevented planet formation

Jupiter prevented planet formation ✓ — Correct! The main asteroid belt sits between Mars and Jupiter, and it holds material that never managed to clump into a planet. Jupiter's enormous gravity stirred that material through orbital resonances, so the pieces kept colliding too fast to stick together and broke apart instead. All the asteroids combined add up to less than four percent of the Moon's mass, which is why we call it a failed planet formation. The Kuiper Belt beyond Neptune tells a similar story, where Neptune's pull kept a planet from forming.

Comets left debris behind — Wrong. Asteroids are rocky and metallic, which fits the warm inner solar system, while comets are icy bodies from the cold outer reaches. The belt is made of leftover planet-building material, not debris shed by comets.

Moons escaped their planets — Wrong. Asteroids are not runaway moons. They are primordial material that never came together into a planet because Jupiter's gravity kept disturbing it.