The rationale is obvious. Ultimately, the ability to exercise physical force decides every dispute. Before you can even have such a thing as currency to exchange for goods and services, you need to have enough rule of law to prevent people with guns from just taking whatever goods and services they want without your consent. So how do you maintain rule of law? Government—which is defined as a monopoly on the legitimate use of force.

Right now, legal tender laws rely on proof of gun. If you owe me a debt and I try to collect on that debt by showing up at your house with a gun to take your gold or bitcoins or whatever, that’s not going to work. More men with more guns will show up and throw me in a cage. So instead of using my gun, I ask the government to use their guns. I sue you, and if I win, the court decides you should pay me, and through a complicated process they end up helping me to your money that you owe me. But the government has a condition: all debts can be paid in USD. I can’t take your gold or bitcoins, unless I have more guns than the government, which kind of makes me the government in the long run.