There are elements of football that do not exist in boxing. Also, I think boxing became less important when athletes started to make bundles of money in team sports. More money and more opportunities to make that money emerged. If football salaries stayed at the same rate as every other occupation (such that guys had to work in the off season), I believe that there would be more guys in boxing chasing the type of money that once was just in boxing. But once guys started making millions playing football (and you didn't even need to be the best like you did in boxing), pursuing a career in boxing became unnecessary. It's a market driven response, not a head injury risk response I believe. Further to my point, the best boxers now are the lower weight categories - could that be because they're not big enough to play football, so they don't have the same access to the big money that an NFL career provides the larger men? Plus, don't forget the rise in the popularity of MMA. Economics and a new (arguably more violent) contest have caused boxing to diminish in popularity. Economics is the primary driver in my opinion. Take away football, and I bet that boxing would see a rise in popularity.