The guardian caps are optional for players to wear during the season. In preseason and practices the oline and dline are required to wear them, plus running backs and tight ends, I think.
Many players will wear them for awhile coming off a concussion. I think they should make it mandatory for a player to wear them for two or three weeks after they come back from a concussion, if not the entire rest of the season.
Players think they look dumb, so they don't like them.