I don't know for recent missions but from a short stint at working with a space agency I remember there were a number of factors. One of them was how long it is from the moment the systems are designed to the moment they are launched. Another was how reliable and well known the hardware had to be, brand new hardware was not a good candidate. Other issues were with radiation resistance, smaller circuits are more sensible, the amount of redundancy that had to be built in and the little amount of power that was really available. In the case of a mission such as this one, I believe transmission is an issue given how far the spaceship is.