The reason people get KOed from being punched on the chin is rather complex. There are a lot of factors/effects that being punched on the chin cause.
Firstly at the point where the mandible connects to the tempular region connects to a series of nerves that can cause the shutdown of the brain, which means one manner in which a person can be knocked out is by shocking the condylaris mandibulae. This is most commonly done with straight punches or hook punches.
A second method is by directly shocking the mandibular nerves (which run throughout the inside of te jawbone). This is the done usually by directly vertical attacks like knees or uppercuts.
The question then becomes what kinds of physical properties prevent this from happening? It comes down to genetics. If someone has sensitive nerves in their skull they are easier to knock out. If the nerves are less sensitive they are harder to knock out. Its that simple.
Sig courtesy of that photo-matic magician limba
I went out the way that I fight. I went out on my shield. Thatís it. Done. --Chuck Liddell
My goal is to end my opponent --Carlos Condit