The laws of the game are constructed in part to limit exposure to head damage. Back when I played (club rugby), concussions were are. Not unheard of - I had at least two of my teammates concussed - one because he persistently led the tackle with his head) - but not common either. This was before the sport became professional and just about when rucks were becoming much more common than mauls.

I'd speculate that the advent of rucks, which are much more violent than mauls and professionalism, which has led to bigger, fitter players, may be driving the apparently increased incidence of concussion,