[Someone]

The explanation I heard for that is that a bare knuckle boxer would be a fool to aim for his opponent's skull, as it would more likely break their hand than knock out their opponent (https://en.wikipedia.org/wiki/Boxer's_fracture)

[cafard]

There is an old brawler's adage that you should never hit somebody in the head with your fist unless you can't find something harder to use.