Mana problems doesn't really have much to do with it (unless you're up against a Priest). The class as a whole simply does not have the tools to accomplish the goals of any of its three roles as such that other classes can not perform said roles far better. They also have far less synergy with other classes across the board, narrowing their possible arena team compositions to a few very select, very specific teams.
I really can not stress this enough: If you are rolling a class primarily with PvP in mind, you do not want to roll a Shaman.