संपूर्ण क्रान्ति है तो जहां है

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!!