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".