This is not an exhaustive list:
- The current way the map is played by horde, which is zerg SH until its capped, then defend it with most of your team while a few people zerg whatever alliance offense exists to get them behind the IWB Choke.
- The map itself enabling that meta.
- The fact that alliance players who queue are generally less geared, lower level, less epic mounts and less optimaly speced
- The alliance incentives dictate that a fast loss > slow win
- The longer horde queue self-selects away casuals by being long
- The HPH being much better in AB/WSG for alliance meaning that rankers queue exclusively either AB or WSG, whereas horde rankers weave in AV games
- The alliance being plagued with more afkers and leechers, without an effective reporting system
These all contribute to the current win-rate and I’m sure there are more. A simple thing such as the horde having a shorter travel time to field of strife, combined with more horde having epic mounts, pvp gear, and pvp spec, while Alliance is only winning in non-participants, means Alliance is both outnumbered and outgeared in that first team-fight. The loss of that team fight starts the downward spiral for alliance until they are trapped behind the IWB choke.
That choke can be broken, but SH GY is the hardest GY in the game to defend for Alliance, and the easiest one in the game to take and hold for the horde. You more or less need to hit SH with a full 40 man using sappers in order to break that choke once it’s in place.