I think they don't want to use CP again as a metric since I recall it used a lot of computational power to have the system calculate the CP of players constantly or something along these lines. But they can limit it to just Archons - Only max level Archons can be used in Diamond/Champ or something. Max level Archons are still a considerable investment so I doubt many (if any) people would buy/rent max level Archons and then fill the team with level 1 cards. Might as well get decent cards while they are at it.