Why Aren't There More Cool Coins?

Sorry, but I'm pretty sure that, if the United States were to do that, it would be in clear breach of the Outer Space Treaty of 1967, Article 2 of which states: "outer space, including the Moon and other celestial bodies, is not subject to national appropriation by claim of sovereignty, by means of use or occupation, or by any other means". All space-capable nations, with the exception of Iran, have ratified the Treaty.

The Treaty prohibits Earth-bound governments from claiming planets, moons, asteroids etc. It does not make any mention of "native" or space-bound governments, so presumably they would be allowed to do so once they came into existence. So to see Saturn on coins, you might have to wait until the Saturnian colonists declare independence.