You want a gas so you don't need to expend energy vaporising the propellant. You also want the gas to be as dense as possible so you can get as much impulse per unit volume of propellant as possible. It's also nice if the gas is inert and non-corrosive so you don't need to worry about it degrading or corroding whatever you're storing it in. Finally it's nice if it is easily ionised.
Xenon fits all these criteria. Its density is around 5.9 kg/m$^3$, and obviously it's inert because it's a noble gas. The first ionisation energy is about 12eV and the second is 20eV so it's relatively easy to produce Xe$^+$ ions.
The only gas I know of denser than xenon is sulphur hexafluoride, but it's only slightly denser and is more reactive. I suppose radon would be better still if only it weren't radioactive. The only trouble with xenon is that it's expensive, but then if you can afford to launch a probe to the asteroid belt you can probably afford the xenon to fuel it.