New champions and ranked system

Why is there not a system in place to not allow people to play new champions in ranked games. Why isnt there something in place to make it so that the new champion is not playable for 2 or so weeks in ranked but allowed in other game modes so poeple can learn the champion. So that way we dont have players inting games in ranked because they played 1 or 2 games in norms with a new champion and they did well so they think they can go into ranked and do as well as they did in norms. Sure you can ban him so that way your team doesnt play the new champion but that takes a unnecessary ban slot.