I think another big aspect of why everyone flocks to Uldah that most people overlook is architecture and accessibility.

Take a look at FFXI. Jueno was THE social/economic hub because it had access to everything. Airships, chocobos, a central location to each city, etc. Jueno was also basically a straight line from end to end, which made it easier for people to find things. San d'Oria was much too big, Windurst was a freakin maze (can't count how many times I got lost there), and Bastok was decent but didn't have all the necessities for travel.

Now in FFXIV, look at all the cities. Gridania is slightly maze-like, and the crafting area is on the opposite side of the map from the market wards. Limsa is just confusing. Having to go from the first floor of the crafting area and having to run up to the second floor of the city is just plain inconvenient.

Uldah, however, is one big circle. The crafting area is near the wards. The Adventurer's Guild is right in front of the entrance. You can find anything and everyone just by running around the circle. It's just all out more accessible than either of the other two cities.