Easy to understand.
Before, bartered goods were "backed" by goods received for goods sold.
There was/is always gold and silver.
Then, there was/is paper money backed by some "government's promise to honor" currency as legal tender.
What does bitcoin have? Besides the faith that "someone will pay a higher price for something of zero intrinsic value"... nothing.
Maybe "digital currencies, backed by nothing" are the wave of the future. Maybe not.
(When bitcoin first caught my attention it had already been to $1,000 and back down to $200. I wondered "what the Hell is it doing even at $200?" Now look. HellifIknow.)