[tt433]

I don't think it's necessarily union control related at all. From an industry standpoint it makes some sense to have a geographic center so that companies who serve the industry have a place to go and find customers, network, etc. I think in this regard it's largely a spontaneous phenomenon more than a coordinated decision. "Birds of a feather."

The other replies to this are interesting though and it's not all one thing, distributed decision making can be rationalized after the fact any number of ways