Because the NFL could "easily" change the rules to minimize the effects of CTE. Easily is in quotes because it's easy except for a fear of losing money.

It really can't. There are many potential smaller changes that could shave a percentage point or two of the risk factor (and the NFL has already made a handful of those type of changes), but the game of football is setup in such a way that head trauma is a guaranteed byproduct. As other pointed out, the cigarette argument is apt. You can make things marginally safer, but there is never going to be such a thing as a "safe cigarette".

It's like a game of gladiators... You can't take the fun off of it.

Because the NFL is making the bulk of the money and they control the game. Who else should be the obvious group to take action here?