Because maybe they are somehow connected with the Torn World. And since that is being presented in Platinum, the creators could not add them in to D/P and so had to wait for Platinum.Maybe they didn't think about it until it was too late to put them into D/P? Or perhaps they purposefully waited so they could add them to Platinum? Obviously I can only speculate, but what ever the situation was I don't think it really matters at this point. What's done is done.