In 2013, in the interest of “player safety,” the NFL passed the one-shell rule that effectively eliminated throwback looks for the NFL by restricting player to one helmet a season. According to this CBS Sports article on the subject:
The NFL says a worn-in helmet is safer to wear than a brand new one that hasn’t yet conformed to a player’s head, which would be the case if an alternate helmet was used with a throwback uniform.
Of course, college football doesn’t share this belief and, frankly, it’s not entirely clear that a worn-in helmet is all that safer than any other when you’re getting hit in the head by a linebacker running full-speed down the field.