It is simply a matter of loco allotment. Traffic density is much higher in the north than south and most of the main routes in the north have been electrified for a long time. So, GZB tends to supply links for those routes.
However, most trains coming from south have a long turn-around time at Delhi or any north destination as the rakes go for secondary maintenance. This leaves their locos, typically LGD/RPM WAP7 or ED WAP4, idle for a long time. So, they allot them for other duties.