Before the release of Burning Crusade, Ironforge was generally chosen as a home base due to its more central location north-south-wise in the continent of the Eastern Kingdoms and it had a direct flight path to Menethil Harbor which was the only Alliance port on the continent at the time.
Some other possible reasons Ironforge is popular:
- The bridge between the Auction House and the Bank is a better landmark than the Stormwind City bank steps.
- Ironforge is built as a big circle so it is faster to get around inside.
- The mage portal landing point is much more convenient to leave from than in Stormwind.
- A flight path to the Isle of Quel'Danas, even though it is a long way.
- Greatfather Winter sits there.
After Wrath of the Lich King, the Stormwind Harbor was built with a direct steam ship connection to Valiance Keep in Borean Tundra, Northrend, so more Alliance started basing in Stormwind City. Although Ironforge still has a good population, you still have to fly to Menethil Harbor to board the steam ship to Valgarde in Howling Fjord, Northrend, so it is slightly less convenient.