Haha, for those of you that don't know, heres why Horde tend to win both of their BGs:
I have played/leveled a Horde Druid and a Nelf Priest to 60. I have played on my alliance friend's warrior, pally, and my horde friend's shaman.
1. Everyone wants to be Legolawls killing the Orcs.
2. Alliance is PvE based, Horde is PvP based.
3. Shamans are different from pallies in alot of ways: if played correctly, pallies can be really really good, or even considered 'overpowered'. Shamans are easier to learn imo.
4. Most kids join alliance, because of number 1.