You're talking about the racials as if it's the only thing that's imbalanced in arena. PvP in general is imbalanced, and specifically arenas. If it wasn't the racials there would be something else to blame. The simple truth is that imbalance is the nature of the game at this stage. In fact, the racials are far from being the most imbalanced part of TBC pvp (and no, TBC isn't the expansion with the worst racial imbalance - that accolade goes to WotLK). Take a look at class balance - are all classes represented equally at the top? Heck, let's not even get into the spec debate... The racials in TBC are certainly powerful, but they are by no means overpowered (NE excluded, although even that has some uses). Ultimately, if you're good enough to get into top 50, your racial won't be the difference between you getting there or not. It will certainly help, but another racial might have helped you in other circumstances.
The more likely scenario as to why most top teams are horde is simply because there are more of them - especially if you consider the original faction imbalance. Most of the top teams have probably been around since the start of the realm (or for the majority of the server's life), so logically they would have gone to the faction where they had the best chance of getting a good partner due to more players. With horde still outnumbering alliance (albeit by a much smaller margin) it's fairly predictable that most top teams will be horde, simply because there are more of them.