Because there is no way to play the game with non-contact. All the other major sports discourage the high risk plays that might lead to injury, focus more on scoring.over defense. Even hockey tones down the physicality. But, there is no way to play football without tackling or playing an entirely different game. I guess you could do a skills challenge instead of a game, but if no one watches the game, no one will watch a skills challenge either.
Edit: Watching the Pro Bowl on Sunday only reinforced my opinion. It was like watching a game of three flies up.