Provable Scarcity - Scarcity, or how commonly something is or is not found, is one characteristic of sound money. The total supply of Bitcoin can be verified and proven.
Scarce Money Vs. Bad Money
As an example, acorns or pine-cones are not scarce at all, and therefore bad at being money. If we used these items as value-exchange mechanisms (money) , then the total “money” supply would be quickly inflated by people collecting them non-stop in forests and groves everywhere.
In modern times, FIAT USD has become the easy-to-produce, inflation inducing item for a few individuals at the top.
Bartering, historically, has been quite popular. Trading what you have for what you need is sometimes the only way of assigning value, or doing price discovery.
Gold & Silver suffer from the challenge of being uniquely useless to humans aside from a few obvious use-cases, modern technology aside. (Relax gold-bugs, I own both)
While some things are certainly more scarce than others, we simply cannot VERIFY how scarce they are, therefore we remain unable to agree on a value for them.
Humans have used many naturally-occurring items as money for millennia, but this changed due to lack of verifiable scarcity. Because we do not know the quantity of these precious items, it is difficult to value these items properly and fairly.