The real question is:
Why do players perform poorly in Epic BGS atm?
People who think that it’s only Alliance who perform poorly are ignorant of the fact that the Horde having a larger playerbase who love PvP contributes to them having players who are more invested in PvP.
There is no such thing as Alliance being bad at PvP. One side has a greater number of players invested in playing and improving in PvP and another side has players who play PvP and may or may not be interested.