1. Most maintenance yards are non-electrified as there will be a need to move equipment frequently across the tracks to attend to the rakes. All the additional wiring and electrification will add unnecessary complications.
2. All of IRs e-locos are dual cab locos and the loco pilot has to switch from one end of the loco to the other every time the loco reverses. Since shunting involves a lot of reversals, it is very tedious and also a bit ridiculous to have LPs running out from one cab to the other every 5 minutes. Diesels on the other hand are designed with a single cab and reversals don't need the LP to run around, thus saving a lot of time.
3....
more... Diesels are preferred for shunting as they are self-powered and not dependent on OHE. Thus they can be operated all the time and lack of OHE poles, transformers etc means that equipment can be hauled around freely in the yard.