It's because in almost all cases, all the damage comes from your brain sloshing around in the fluid inside your skull and ramming against the front of your skul

It's because in almost all cases, all the damage comes from your brain sloshing around in the fluid inside your skull and ramming against the front of your skull when your skull suddenly stops moving, it doesn't have a lot to do with surface trauma to your skull from the impact itself or anything like that. Until we can wear a helmet that directly augments the connective tissues inside the skull, we won't be able to solve concussions with head protection outside the skull.