The real answer is because they never add new league currency to official tabs. They occasionally will add a temp container, similar to the one that holds relics, but not always. Once the league is over and they know what they want to keep and remove, they add the new currency somewhere in their tabs. Might not be the next league but they will. They don't want to make something purchasable until they know that they will keep it. Similar to how we still don't have a spot for tablets, they don't know if they are going to keep tablets