The main reason they don't is they can't put the rabbit back in the hat - it's a permanent alteration to the game. They also worry that cross-play might cause the name of the WOW TBC Gold horde/alliance being lost and that's a problem considering how much marketing it generates.
Details are the key to success. When you look at the PvP servers and the balance is totally wrong. 40 40% alliance, 60% Horde. In PvE it's the reverse opposite: 66 percent Alliance 34% Horde.
So what's the reasonable conclusion? Players on the alliance side are more PvE-oriented and that is apparent in server choice and side distribution. Players on horde side are more PvP oriented. This results in a huge increase in queue times despite the two sides being equal in general.
With regard to 1, we are able to reasonably from the possibility that it will not be a success. Free transfers away from full servers with massive queues were not used. A slight shift in racials did not convince players. It is acknowledged that the arena alliance racials are superior if the other elements are equally. Horde PvPs are eager to be Horde. It is understandable.
I said it's an ineffective argument since it's a simple matter to avoid. If the population of servers in a PvP server is over 55/45 Horde is the most popular, and making it impossible to make a new character on Horde is a fairly easy way to get rid of the server being massively imbalanced.
The current approach is a blunder on the whole concept of the game with two factions. It's been the basis of Warcraft since its inception. A better solution would be to offer free faction change (mindful it is possible to limit the amount to one account per player), so that people are able to swap their entire characters.
It seems like a straightforward solution to stop the creation of new characters on horde but the implications and the backdraft would be massive with plausible arguments such as "I would like to play with my buddies" or "I would like to be a the horde because I love orcs."
You would also have to limit the creation of cheap Burning Crusade Classic Gold Horde-related characters on all servers within the bgpool. It's not really feasible, or is it?