I'm totally speaking as a guess here, but I would think that it would take a lot more processing power and improved circuitry for that to happen inside the monitor itself, on that little digital processing PCB. As I understand it, on TN panels you have a 6-bit process, so the info is quickly converted into that format, and it's easier to feed that much smaller gamut into the panel at a high rate of speed. With IPS, you are running 8 or even 10 bit panels, which doesn't sound like a big difference, but actually is a tremendous increase in information out to the panel. This would seem to require a lot more out of that monitor component. I know less than nothing about who makes those processors, but I would hazard a guess that it's the magic that needs to happen to make 120hz doable with IPS. Some models can actually overclock to 100hz and beyond, but with varied results in reliability/longevity. I think the actual pixels on the panel itself can handle it, but the limitations before that signal is generated is where the problem sits.
I'd welcome more info on it for sure

All of the above is a somewhat educated guess.