Less to do with individual player skill and more to do with teamwork. The horde are much better organized. Faction pride or IRL friends or what ever. Most Alliance players don’t really like play with other people. When they do it’s usually a means to an end. “I need these fools in order to get the raid loot I need. Please dear god keep the repair bill down tonight!” or “Heck no! I’m not following this kid into that obvious kill box. How can they not see this as a trap?” The fall of the Raid guilds to LFR and LFG in retail are proof of this. Horde is awesome so long as there are more of them than us