The reason that they aren't using different currencies - at least at the moment - is because money is a physical item in the inventory, and right now there is no system for say, dragging 35 copper pieces onto a pile of 21 silver to create a pile of 23 silver and 5 copper, or for taking 3 silver and turning it into 30 copper.
In theory such a system could be designed, but I don't know if there are any plans for implementing such a system. They might not think it worth the design effort to implement.
Such a system also needs to take into account all sorts of potential issues with closing the inventory or the player in a multiplayer game suddenly being disconnected, which may or may not be added complexity compared on top of normal inventory management.
