Reduction in SL and GN coaches is happening all over IR. With introduction of AC Economy class, more SL coaches are turning to the noose. Many trains now have more AC coaches than SL. This is despite the fact that still, pax prefer SL over AC on those routes.
I assume this is due to government's gradual push to shift IR slowly but steadily towards more air conditioned trains and less of SL/GN travel; especially on overnight routes.