Don't know the stats, but I'd think that with the improvements in strength training, in sports in general, but especially in boxing, fighters are getting hit harder, therefore taking more 'dangerous' headshots.
There was a move to try to ban boxing some years ago. Don't know if that'd be the answer, with all the money it generates now. And apart from blows to the head being disallowed, as in some martial arts contests, I can't think what else could be done.
Be good to know the stats of ko's by headshots from say up til the 70's and to the current day