Dave said:
"I suspect (intuitively) that distance from the sun is a big driver of the ability of a small 'planet' to retain moons. Here's my (Newtonian) reasoning."
Also check out:
http://en.wikipedia.org/wiki/Hill_sphere
Even though the gravitational attraction of Pluto is far more feeble than even our moon it is able to hold on to more orbiting bodies due to the simple fact that the gravitational effect of the Sun is also weaker in the outer periphery of the Solar System compared the inner Solar System.