Well at its very core, the squeeky clean image of the Alliance races do appeal a lot more to younger players. You have the token human race, with the knight-in-shining-armor image, plus the night elves and dwarves to bring a LOTR influence to begin with. There's also no doubt that gnomes have a childish stigma to them.
It's the complete opposite for the Horde, which have designs based off far more mature origins. Taurens are influenced off ancient Greek mythology, undeads are just a bunch of zombies, trolls and orcs are equally vicious and been in far more mature and violent tales to begin with. Orcs in most other tales typically have a backstory of rape, murder, pillaging and destruction.
That's the most logical view to take, pre-tbc, but now both sides have their polar opposite races - the blood elves are the 'normal' race in a mostly-savage faction, while the draenei are the strange, beast-like addition to alliance.