Original: Since tidal forces overwhelm gravity within the Roche limit, no large satellite can coalesce out of smaller particles within that limit.
Simplified: The Roche limit, sometimes referred to as the Roche radius, is the distance within which a celestial body, held together only by its own gravity, will disintegrate due to a second celestial body's tidal forces exceeding the first body's gravitational self-attraction. Inside the Roche limit, orbiting material will tend to disperse and form rings, while outside the limit, material will tend to coalesce.