Once all classes and specs have a chance to beat each other if played right, then the rest of the game is ready to be designed around it.
You can't do it in reverse, to first create PvE bosses, then balance classes to be about equal in dps/healing meters and then tweak them because of the pvp. That can only work for one patch, then it has to be done all over again. If they would do first balance classes and specs around 1vs1 then they would only have to balance it once per expansion and not having to waste time in the future.