It is very simpel.

Like the Oil price is coupled with the Dollar, they coupled the resources to Vespene.
So you do not have different lots for Metall, Mineral and Vespene.
 
 
 

If you could move Space Stations, every big Player had one nearby.
Or the big Alliances would create some Systems, specialist in ship building
near their center.
So no chance for a normal player to get one.