less hair = less traction when slipping out of submissions, moisture evaporates faster off a bare scalp cooling you more quickly, no potential for "accidental" hair grabs, shaved hair never gets in your eyes, and even if it's just an ounce, it means less weight to cut. so from a competitive standpoint there are reasons to shave your head.
Same reason why some dudes shave their body hair too, and/or their faces, friction. Having thick facial hair just adds friction when a fist hits your face, reducing the chance that it'll slide and turn into a glancing blow.
And yeah, some guys just look super fly clean cut.