Darjeeling is not the only tourist spot of this route. This is for connecting Northeast to main India.
By the way, for the route VSKP-GHY .... tourists have lots of daily trains to make way by breaking like ... VSKP-HWH ...HWH/SDAH/KOAA-NJP
And tourism is not the only aim of IR,if it was so, there were no train arrivals or departures at NJP from 15:00 hrs to 6:00 next day. Is it so?