Balance.
First and foremost the game is balanced around PvE because that's simply what Blizzard decided WoW should focus on since it was in development, because of this the game will never be balanced around PvP ergo PvP will never be balanced.
PvP in WoW wasn't even going to happen at all until people started requesting it, then it turned into BG's and then Arenas. PvP has always been something that was tacked on, it was never meant to truly be something a person can main.
PvP was always designed to be something you can do on the side. Unfortunately people inevitably decided that they wanted to main PvP and thus take it seriously, despite the fact the game had never intended for there to be PvP in the first place. These people will bitch and whine that PvP is never balanced and that PvP is ignored etc etc despite the fact they know (or don't and should) that WoW was, is and always will be a PvE game.
In a nutshell? WoW has always been a PvE game, not a PvP one. PvP will never be balanced nor will it receive any where near as much attention. Accept this or move on.