Not really seeing it.
Ironforge being the main Alliance hangout is pretty easily understood. The city is the closest one to BRM and MC/BWL, since the combined flight time from there to Thorium Point, and then to the instances is noticeably shorter than flying from Stormwind to Morgan's Vigil and riding northwards. Once Naxxramas comes this will be even more notorious.
On top of that, the anvil/forge areas are pretty close to the main Commons AH/Bank part of the city, and the circular design makes the whole layout quite compact.
Seems to me that following that logic, Horde's preferred hangout would be Undercity. It's the only capital in the Eastern Kingdoms (BRM raids, and the closest city to Naxxramas) and therefore the only one that avoids you having to wait for a zeppelin to cross (a zeppelin that would leave you at UC's gates anyway), you also get a Grom'gol direct connection when doing ZG, and in terms of layout, there's also a lesser distance between the perimeter's AH NPCs, the bank at the center and the flight master. The circular compact design shows this.
On the contrary Orgrimmar doesn't seem to be closer to anything, even if AQ is considered Thunder Bluff is the nearest city, and having to go to the deepest part of the Valley of Honor to smelt or craft blacksmithing/engineering items, then having to go all the way to the Valley of Spirits to level first aid, and then returning to the center section for AH/bank seems to be a longer while.
Is this because of the earliest patches when (if I recall correctly anyway) there was only 1 auction house per faction, situated in Ironforge and Orgrimmar (reason why only these two have proper buildings, while other cities just have some makeshift location) ? Or why?