Money we are using now is backed by obligation of people to pay it back and very well developed institutions that enforce it. If you hold 100$ you know someone, somewhere needs it to pay their debt. If you hold Bitcoin no one ever needs to buy it from you unless they hope they can find a bigger sucker later.

Instruments in money market are of the kind of: "give me 100$ now and I will give you 101$ 3months later". Bitcoin is not backed by such contract. It's only backed by the hope of finding a bigger sucker later.