As we know, the chin and the jaw are major weak points when it comes to getting hit. Other weak points present are the genitals, the eyes, the kidneys, and the liver. These are all sensitive organs, but the chin and jaw are made of bone. What makes it a weak point is it acts as a leverage point to move the head quickly with little applied force. This allows one to easily rock the brain.
There are some people who just seem to have iron jaws. It is a combination of neck and bone structure that somehow allows them to sustain more punishment before their brain decides to shut down. However, it may be just that their brain has a higher threshold, as many iron chinned boxers in history would still suffer severe brain damage towards the end of their careers.