They clustered in Ironforge because it had the AH and was more central on the EK continent. Once the emphasis was removed from flightmasters and auction houses were added to other cities, people naturally clustered in Stormwind instead. It was split a bit evenly but people did seem to prefer SW over time.
No one went to Thunder Bluff, meanwhile, and Exodar and SMC were mostly empty since their creation. Darnassus and Undercity has similarly light populations - usually just undead and night elves moving through, or, earlier on, people who had business on that particular continent.
Once the bulk of players choose a place to congregate, everyone else goes there because they like being surrounded.