हमसे सीखे कोई जीने का सलीका। सफर कैसा भी हो, मौज़ उडाये जाते हैं।। - Amir

yep, totally related to electric machines and electric drives ;)
when you go for v/f control, your motor has 3 'zones' of operation: Constant Torque region, Constant Power region and High speed region.
in Both P7 and P5's graph you'll see a flat region in the beginning. this is constant torque region, where we maintain v/f ratio constant, which means torque is also constant. As you must have guesses, as speed increases, frequency of rotation also increases and hence to keep v/f ration constant you have to increase input voltage to motor. However...
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you can't keep on doing this for a long time as every motor has winding with insulation, which can tolerate an electric potential up to certain level only. once that is reached (2180 V, for both locos ) you can only increase frequency but not Voltage. this is the non linear dropping region you see, where torque drops as v/f ratio falls. Eventually though you also reach a point where you can't even increase frequency and you go for field weakening. that is the last, sharp drop region where troque eventually drops to zero. for P5, the point of 3rd region arrives later than that of P7, as its RPM is higher, hence you get to play in the higher torque region for longer time than P7 (160.3 Hz for P5, compared to 132 Hz for P7)
PS: I assume you know about v/f, without which my answer will read like Greek to you. The assumption is based on solid grounds though, i know you are well read, curious person so its unlikely you aren't aware of v/f control :P
Image source: click here

One reason is the space constraints due to the new transformer, as mentioned in the other blogs. This was a problem at that time, because, even though transformer taps were provided, a converter still had to be installed in the locomotive to be able to connect to the transformer and supply the power to the rakes. Usually, the transformer tap+ converter combo is decided based on the maximum rake length ( i.e., maximum power drawn by the rake) that the loco has to handle. Since the passenger trains in India are usually much longer than their European counterparts, the original design was not sufficient to power the longer Indian rakes, and an upgrade was required. This was easy in the WAP7 as it was a longer loco than the WAP5 and had enough space.
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Another reason is that IR took a lot of time to adjust to the 3 phasers and the LHB rakes. Initial trial runs of the LHB coaches with the Lucknow Shatabdi didn't go well, with a lot of jerks and ride quality issues. While this was going on, the P5 was struggling on many routes due to lack of track maintenance which was leading to a lot of gear box failures. At the same time, the WAP7 was suffering from heating and brake issues, while the engineers were struggling to optimize it to have better acceleration than the WAP5. With such critical problems in their hands, they didn't pay much attention to HOG ( Though why they didn't do it at that time for WAP4 is a total mystery !! ).
The funny thing is that HOG was a regular feature during the steam loco era, and now almost 40 years after the steam era, we are still trying to re-implement it!!