WAP 7/5 can also fail, courtesy their power elctronic devices which are prone to failure in high temperature in India.
compared to P7, P4 are also low tech (have nos sophisticated control circuitry) and thereby easier to understand (as you can in many instances physically see the wires and relays doing various tasks) and repair, compared to three phaser locos.
but efficiency is low, i'll admit and is probably an important driving factor for this decision. they have poor power factor too, which leads to line losses. also, with every new generation of...
more... induction motors the efficiency is improving even further (though IR hasn't adopted them yet)