Because the land is owned by the U.S. Federal government after making deals with European countries, Native Americans, and also taking it from Native Americans by force.
No one has yet transfered the property rights to the land to anyone else.
Edit: If you really believe the freemarket is the best to determine what is the preferred mode of exchange. Then decentralized currencies are inevitably going to win out, especially if they're equally accessable. (Meaning no need for extensive tech knowledge or expensive mining rigs).
A currency that has a built in UBI and virtually no transaction costs is going to be attractive to consumers. And merchants will eventually gravitate towards it more as they see it enables consumers to consume more.