Unsubstantiated, un-sourced theory: football players are helmeted, and that helment makes them feel as if 1) they are risking no head damage to themselves, and 2) there is no risk of head damage to others. Thus, lots and lots of targeting of the head and/or usage of the head as a lead component in making or breaking tackles.
On top of that being probable, players try to hide concussions (resulting in compounded head injuries the next time), and players rebel against the NFL's attempts to legislate fewer dangerous head-related hits and tackles. Actually, even fans and the media have the tendency to pine for skull-crushing, cringe-inducing hits at high-velocity.
I could probably have summarized the problem in one word: "culture".
On top of that being probable, players try to hide concussions (resulting in compounded head injuries the next time), and players rebel against the NFL's attempts to legislate fewer dangerous head-related hits and tackles. Actually, even fans and the media have the tendency to pine for skull-crushing, cringe-inducing hits at high-velocity.
I could probably have summarized the problem in one word: "culture".