It’s because they don’t open up and fragment like the eldm/tmk bullets. The guys chasing the smaller diameter less recoiling rifles in general want to get into the lungs/heart and rapid expand to disrupt the Circulatory & and respiratory system as much as possible so the brain is starved of as much oxygen as possible and dies. Because that is ultimately how animals die.
My experience with them is quite opposite. The forward 1/3 of the bullet fragments almost immediately inside the animal. Usually there’s a hole, and there will be 4 or so smaller holes in a pattern around the main exit where the fragments exit as well. The insides are absolute soup. I haven’t shot the HHT yet, but the pics I’ve seen on their forum are pretty impressive as well.
What you are referring to I have seen from Barnes bullets. These bullets are a harder copper that fragments.