This is how I see it:
1.) There are more players Horde side that PvP.
2.) There is an anniversary quest tied to PvE-style quests in AV.
3.) On Alliance, you get a higher proportion of players who are there more for the mount then they are for the PvP.
4.) Horde got the mount quicker and now have a higher number of players there actually for the PvP and not the mount.
The fact that there is so much talk about Snowfall Graveyard goes to those points.