This is because it shares a slot with Andaman/Himsagar/Navyug expresses between NDLS and JAT. You can see that all the above mentioned trains too have long halts in the Delhi area.
This is a relic from the past. Earlier, all these trains going to JAT were used for military personnel. In addition, due to single line sections on the route, NR preferred to run all the trains on a similar slot.
They may block a platform for a long time, but then running all 5 trains (Andaman, Himsagar, Kota-SVDK, Navyug and JBP-SVDK)...
more... on the same schedule for the whole journey till SVDK makes NR's job much easier.
Ideally, it would have been better if all of them share slots over as much of the route as possible, instead of sharing it just between NDLS and JAT, but it may be difficult as it would need coordination between many zones on busy trunk routes.