Ah, the comparative weight thing. Pull off the solar panels, their associated PMADS, and battery, and stick in the RTG, and its PMADS. The difference? Probably not much. Remember the ALSEP RTGs were designed to be carried by people (albeit at one-sixth g). My guess is that the engineers could have made it work, if the choice were made to go that way. I'm thinking maybe something other than science was involved in this decision.