[dr_orpheus]

They need the milli-newton level of thrust control and so need tiny thrusters. Generally the simplicity of the cold gas thrusters in a small package is easy. Yes it is not as efficient, but moving up in efficiency and complexity to catalyzed monopropellant (generally hydrazine) thrusters the thrust ranges are usually > 1N. Same story with any bi-prop thrusters. Certainly electric propulsion has the levels of thrust needed, but then you would need a lot of power for it. And since they are not doing large delta V maneuvers there is less concern about the actual amount of propellant that you need to take with you.