It isn't the helmet that's the problem. As their ability to absorb shock has increased, the players hitting harder has gone up with it. And although a helmet can absorb a lot of shock, the twisting/turning of the head on impact is not something they can dampen, and those are the motions that cause brain trauma.

Keep in mind that the interior base of the braincase is a catastrophe of sharp bone, and that you're scraping something with the durability of jello across it every time there's an impact that has any lateral component, which is all of them.