Upkicks to the head of an opponent on their knee(s) are illegal rather than being separated out from other kicks and knees to the head of a grounded opponent not because the upkicks themselves are so terrible but because if they were legal then any fighter in position to take such an upkick and with above room-temperature fight IQ would have to be so busy defending the upkick with their hands that they couldn't engage on the ground, and that would make for unwatchable fighting. In the early days of MMA even "elite" fighters mostly had no fight IQ, so this didn't matter so much, but today it would be truly awful to watch. Almost no one would posture up for G&P, no one would jump into an opponent's guard, and when fighters ended up in the kind of position where upkicks to the head could happen it would be like watching a slapfight until the ref stood them up.