User blog comment:ChishioKunrin/The Schnee Dust Company's treatment of Faunus workers/@comment-25368963-20150118051529/@comment-24198837-20150118163147

Agreed with Dark and Chishio. The first attempt failed horribly, so they tried again differently, and again and again untill they had sucessfully expanded. And they're like to keep trying - they can't not. The only alternatives are  "poplation control" (Which can get very dark, very fast) or space exploration and colonization, and I'm not sure RWBY's going in that direction.

It's possible that all of Vale's districts are technically all expansions, with the walls of the old city in between.

That would also explain why the center of the city, where we've been currently, seems to be higher end and wealthier - it's the oldest part.