The temperature in this part of the nebula was just right to prevent rock from sticking together. Gravitational tugs from Jupiter halted the growth of a planet and initiated destructive collisions. A planet once formed there but was ejected from the solar system. Asteroids formed inside the frost line while comets formed outside. Asteroids and comets formed at different times.
Comets are much larger than asteroids.
Asteroids are much larger than comets. The orbits of comets are quite different from those of asteroids. Pluto's orbit is completely outside that of Neptune. Pluto's orbit is completely inside that of Neptune. Pluto's orbit crosses that of Neptune and they will one day collide.