I never quite understood this myself, UC has a better layout, similar to Ironforge.
The closest cities from the perspective of raids (and D1 sets):
Scholo/Strat: Undercity, Ironforge
BRD, BRS: see T1/2
Dire Maul: Thunder Bluff yaaay, uhhh.... well Darnassus offers direct flight path to Feathermoon, I guess. Or you can IF -> Menethil -> Dustwallow -> Feathermoon.
ZG: Stormwind, UC/OG (tie, as both have zeppelins to Grom'Gol and both of those are outside)
T1/T2: Ironforge, UC (sort of tie, UC offers a direct flight path, while OG requires boarding a zeppelin)
Onyx: Orgrimmar, Ironforge (through Menethil)
T2,5: TB/OG, IF through Menethil and then fly from Theramore to Cenarion Hold (still closer than SW ->BB ->Ratchet ->flight)
T3: Undercity hands down, Ironforge.
Undercity has the main advantage of PHYSICALLY being on the same continent as 4/6 of raids, so you can just click on the flightmaster and go make yourself a sandwich.
Then again, UC originally didn't have auction. OG had, houses two races and is on Kalimdor, which is mostly considered a Horde continent as the balance of cities is 2/1 for Horde and TB is very close anyway. Except for the forests. Orcs may be green but forests ain't their friends.
Plus, OG has the rugged feel of wild, untamed land. I always welcomed an opportunity to ride through the steppes and wastelands of Kalimdor's wilderness, as those places breathe something vast. It's just plains and mountains stretching for gigantic distances in every direction, and you can very easily get lost there. And Orgrimmar just feels like it's a natural part of this.
Which frankly just makes it a nicer place to stay in.
Funnily enough, on the free TBC server I started on, the Alliance "capital" was always Stormwind. I dunno if this shift happened on retail TBC too (no reason with there being Shatt, really, same on wotlk with Dalaran), but for SOME reason The Snow Blowers decided on Cata that SW is now the capital and that Alliance = Varian Wrynn.