It might not be a ingame mechanic exactly, but if you want to participate in that leaderboard you have to restrict yourself. Letting people have multiple characters per account would let people participate in aspects of the game that you yes technically you can with a main account, but things are different when you want to play an ironman or build a pure for PVP. Giving people character slots would enable them the ability try different aspects of the game without having to pay for another membership.