Its simply, because,
1. Old train get new stoppage induced, making them slower
2. To compensate, new train are introduced, pushng old train backward
3. Old train offer passing/crossing facility to new train, hence become last option in IR Priority Book & Passengers' Journey Book
4....
more... Trains (no matter old/new/fast/slow/dirty/clean/late/in-time/unearthy hour) are always full (except few exceptions) & IR counts this as trains success & so become neutral or apathetic towards Better Treatment!!!!
5. Ministers are busy offering n nurturing their own new babies (DURANTO/SAMPARK KRANTI/GR/NEW SUPERFAST), forgeting mature old junkies!!!!!!