Gold has a couple other things going for it. It's difficult to counterfeit, anything denser than it is more expensive(and usually radioactive). Tungsten is close enough to be a viable alternative, but is very difficult to work with, so this is only a modern (1980s on) problem. Additionally it is "appropriately" rare. Try using sand as a currency, you'll need a fleet of dump trucks. You can transport a significant amount of gold on your person, but if you tried that with Californium you would probably lose the dust sized particle that represents your life's savings.
