Basically they don't have the development time to balance 2 sets of skills.
BUT in order for this to occur, PvE and PvP would need their own skills. It can be the same skill names but the effects and use have to be game mode specific.
It's an obvious direction but one that will be as huge as One Tamriel....hoping it happens in 2017
Vicente Valtiere, Dark Brotherhood, OblivionSpill some blood for me dear brother