In general, I think the other 2 cities are dying because people just have an urge to congregate. Uldah is the chosen spot because it's the center city.
Other factors include it's selection of guilds. All crafters need weaver repairs, and lots of them.. Well, a good place to get them is the city with the weavers guild. Crafters also usually rely on alchemy mats, and it has that guild too... And goldsmiths are very helpful to repair weapons and jewelry, especially for lower level chars since the repair npcs skip fixing your jewelry... Well, uldah has that guild as well. Sure the other guilds are nice too, as are the other crafters, but they all gather in uldah for the same reasons as everyone else.
The location of the camps is probably a factor too. I know that's where I did leves at 33+, because you can just hike to broken water, do leves, hike to halatali, do leves, then use your last leve to tele back to drybone, and it's a short hike to town. Now with the whole favorite camp and town anima reduction, that's not a big deal since it's only 2 anima to go to a city if you're in that zone, but it's still more convenient then hiking to nine ivies, and then waaaaay the **** over to treespeak, with practically no chances for autorun.
But the biggest reason, is the selling aspect. People like to have a central place to buy things. Early on they shouted in uldah, and now we have the ward search, but either way... At first uldah only had a slightly higher population, but it was enough to make people start gathering there. That's why I left grid, even though I do love the look of the forest and all that. But hey, the wards are a stones throw from the aetheryte and adv. guild, and the other guilds aren't far either... The city is just built to be ugly but functional and convenient, and that's tough to turn down after everyone else is already there. And if anything, uldah is just filling the void that the lack of ishgard/jeuno has left. When that comes along, uldah will dry up quick, because everyone will pack their retainers into the ishgard wards, and set that place as their homepoint.
As a side note... I love the forest areas in the black shroud. That and coerthas are by far my favorite zones aesthetically... But I agree the whole maze layout sucks. If it had a much more open area style, but still had all the trees, it would be so much cooler. The tunnels really kind of ruin it though, you just don't feel like you're roaming through a forest, it's more like a trench most of the time. The spots that open up are nice, but they are too small and uniform because of all the copy pasted terrain to really make up for the rest of the zone. Overall I would love to see a second attempt at a good forest zone, along with a snow zone... But grid as-is... meh. Just meh... For so many reasons. But, next time you're in the area, try going into first person mode and looking straight up... And stop to poke around some of those flower patches... They really did a great job on the place aesthetically, at least in part. Until that unavoidable mad angler agros your lvl 15 botanist, then the long trench-hike sort of dampens your appreciation of the pretty graphics.
At times it feels like some overly tired developer was toiling away building the zone, and after placing one too many bushes, suddenly decided the place should be like a giant hedge maze, and nobody stopped him. ;/