since the start of wow horde was ever the stronger side. Its simply because people like to be on the "bad" side, on a side that is not gay washed fantasy, the side with the more badass lore and ncp characters. Look up WC3, the horde storyline settled the hype.
Racials affected the ballance, sure. But not in tbc, in woltk the human racial is so powerfull that hardcore pvp players switch to human. Same effect was back in vanilla retail time when people realised that UD racial was that strong. In TBC UD aint that strong any more. The faction inballance is simply because people like horde moe than alliance. Many old palyers who played back in retail come back to enjoy tbc once again, most of them stick to the faction they played retail. retail servers had 75/25 rates or even worse, if you want to enjoy a blizzlike server, that what you get.