Blizzard didn't have time to do the architecture work to make auction houses linkable across servers (as different regions of the world, even on the same continent, are on different servers, this is required), and since multiple auction houses just ended up with everyone ignoring two of them per side anyway, they went with the current solution.
They ARE now working on setting up auction houses in each racial city that will be linked together. Once one can be in Stormwind or Darnassus and access the same auction house, I think we'll definitely see a decrease in the number of people there. In alpha and beta, people congregated in Stormwind rather than Ironforge, and once they have the option, I suspect some people will move there again. Darnassus is a different story, but having the city be more useful should still ensure that people adventuring in Kalimdor at any level will swing by there -- if Darnassus and the AH is a short flight away, suddenly it makes sense to bind your hearthstone to Everlook, for instance.