Loco links are usually allotted in such a way that there is maximum utilization of the available locos. While a P7 is a better loco than a P4, the difference is really not much. The P4 itself is such a good loco that even in terms of acceleration, the difference between P4 and P7 isn't much. Other than a handful of trains ( like BPL Shatabdi, Gatimaan etc) pretty much all trains can maintain their schedules with any of the WAP locos or WDP locos. Allotment is just a case of utilization.
Many Kerala bound trains continue with WAP4 links as ED has lot of these locos ( highest holdings of WAP4 in fact). So, no necessity...
more... of working out complicated LSA arrangements to provide a P7 when there won't be any difference in the run time.