topic from 2006. Radioastronomer is gone now.
There is not enough mass in the asteroid belt. Jupiter played a large part in the prevention of the asteroids from forming even a small moon like object.
There is not enough mass *now*. Jupiter has more than half the mass of all known Solar System objects in orbit around the Sun (IOW, not counting the Sun), but is enriched in noble gases, so (from the standpoint of the dominant model for planetary formation) either it formed much further out, or accumulated most of its mass from interaction with other bodies.