Because its irrelvant to forming the solution. It looks sexy and try's to make the solution look aero dynamic, but that is the sum total of its usefulness.
All you do by bring TAS into it is introducing a heap of enviromental variables. Which then cancel each other out because your comparing aircraft in exactly the same situation bar the fact that there weight is different. Again its irrelevent to the solution apart from it makes it look aero dynamic.
You are trying to give an expicit answer to something that doesn't require one.
The point I am making is that if you start bring L/D ratio's and other such factors into play to answer the question. You will just come across as using techno waffle to answer a simple question.
So my answer would be
"The heavier aircraft has more potential energy than the lighter one and at a constant airspeed they both remove the same amount of energy via drag in the decent. Therefore for over a period of time the lighter aircraft would have lost more altitude than the heavy one ie the lighter one has a higher rate of decent"
Thats the answer they are looking for which for wannabies is the answer they should give if they get to a oral tech interview.