Among other things they typically have a small mintage.
Read about the Carson City mint at this link:
"For numismatic devotees, coins of the Carson City Mint hold a special appeal. Long associated with romantic Wild West imagery, the thrill of owning rare coins made of gold or silver mined from the Comstock Lode is a slice of Americana every collector dreams about. Thanks to its unique place in history and a rich heritage of "CC" coins bequeathed to this and future generations, the memory of the Carson City Mint will rightfully live for ever more."
http://www.us-coin-values-advisor.com/carson-city-mint.html