Are we sure it's not prevalent? Concussions & CTE weren't widely reported in the NFL until recently. Googling for "rugby cte" turns up articles like this one (http://www.telegraph.co.uk/sport/rugbyunion/11857548/Rugby-u...), which suggest that it might be more of a problem than you're implying. It's even a problem in non-American football/soccer (source: http://www.businessinsider.com/soccer-heading-brain-damage-d...).