They can't, because they're unable to put the rabbit back into their hats. It's an ongoing change to the game. They also fear that WOW TBC Gold cross-play could result in the identity of the alliance/horde becoming unimportant and that's a problem given the amount of marketing it creates.
The details are what make the difference. The balance of PvP servers is wildly off. 40 percent alliance, 60 percent Horde. PvE is the reverse 60% alliance, 66 percent Alliance, 34 percent Horde.
What's the conclusion What is the conclusion? Players on the alliance side tend to be more focused on PvE, and this is evident in the cheap Burning Crusade Classic Gold server selection and side distribution. The Horde side is more PvP-focused. This results in an increase in the time it takes to queue despite the two sides being identical overall.