I don't know the actual technical reason behind it, but I guess one active EOG is enough for LHB trains like Puru and Mahabodhi with 5-6 AC compartments.... but Rajdhanis , Shatabdis, Garib raths, AC Express trains and AC Durontos need both the EOGs running as these are fully Air conditioned trains..
But again there is a question in my mind that why in Puri Shatabdi also the rear EOG is being turned off most of the times ??