Why "should have"? Part of the reason this series has been so entertaining is because Paul and Harry have been so dang good in their roles as traitors.
Meanwhile, some of the women have been so anonymous and pointless that the only time you've really noticed them there is when they've been murdered or banished.
No doubt the BBC will urgently add some ridiculous new rule that there HAS to be one female traitor in the game at all time for diversity's sake from series 3 onwards, and cock the whole thing up...