Why don't more pro football players talk about their injuries and cognitive problems?

Taboo subject. No one wants to make people aware of their shortcomings, especially while their careers are active. We are seeing more and more retired players speaking out about things like that, and thanks to them there are changes to make the sport safer. It's a slow process to make changes to anything that's been around a while.
Good Question. Players are bigger and stronger than in the past so the hits they experience are at much greater forces. I think you are seeing more and more players speak up, especially with the "bounty" story involving the new orleans saints. Fortunately more research is being done in this area; such as the best shock absorbing material for a helmet. I think you will see more speak up.