User blog comment:ChishioKunrin/The Schnee Dust Company's treatment of Faunus workers/@comment-25368963-20150118051529/@comment-4010415-20150118155154

That's pretty much what I thought, Darkcloud.

They didn't return after their initial failure because they really needed the expansion to be done. Oobleck said it was Vale's first serious attempt at expansion.

Keeping the entire world's population cooped up in four kingdoms means they have to make expansions whenever the population rises too much.

Maybe the reason they used subway tunnels for the transportation instead of flying is because of flying Grimm like Nevermore. Or, for all we know, it could've taken place before they perfected public air transport.