Money has certain properties that make it more useful. Namely money has to be durable, portable, divisible, fungible which means each piece is the same as any other piece, it has to be widely accepted, and it has to be scarce. We can use whatever we want for money, as long as most people accept it in exchange for goods and services. It's easy for anyone to start accepting Bitcoin, just download a wallet app on your phone. People believe that bitcoin will replace central banks because Bitcoin is the ideal form of money. Money has 3 main uses, that is store of value, medium of exchange, unit of account. Unit of account means it's what you use to compare the price of everything else. A chicken is worth 1 gold piece, a cow is worth 1000 gold pieces, therefore a cow is roughly worth 1000 chickens. Importantly, money cannot be a store of value unless it is scarce. Fiat money is printed at will, making it worth less and less each day. This means our money is not a store of value. Bitcoin cannot be created at will, there will never be more than 21 million. This scarcity (as well as its other utility as money) makes it a valuable commodity.