It's a gear issue, for one. The vast majority of Alliance players are players leveling up. Meaning that their scaled gear is absolutely dreadful. Usually 4k-5k health. The max level character I did AV on is not exceptionally geared, and I go in with a scaled 7k~ health. As a DPS. And when things like "how sucessfully can you perform your role when fighting NPCs" is a major obstacle, gear itself becomes a problem.
Why might this disparity exist? Probably because the Horde know they can queue up as max level characters and actually win conquest and time-warped badges and all that stuff. While the only Alliance left doing it are those that try and AFK to level up.
If they wanted a "balanced" AV, they should put it in brackets... a bracket for players leveling up, and then a bracket for max level players getting scaled down to 60.
There are a few other issues as well. "The bridge" is not nearly as insurmountable of a barrier as Hordies who whine about AV on live make out, especially not compared to the layout of Frostwolf village, and ESPECIALLY when you factor in the NPCs actually providing a realistic barrier.
It's a straight shot from SPGY to the Aid Station, with the only real choke being the bridge itself... which the Horde respawns right on the other side of, without any intervention from Alliance NPCs. Not remotely the case for FWGY to RH; the distance is far, FAR larger, and has three chokepoints the horde can feasibly turtle up in. The layout of the village also allows horde NPCs to periodically path in and out of the area and respawn, meaning that people rezzing at FWGY periodically aggro horde NPCs in the intervening village.
And while most of these things wouldn't be issues, the dearth of geared players absolutely becomes an issue when it comes to turtles or trying to outzerg the enemy team.