Earlier quoted context omitted.
The big difference is the FDIC.
Which, if it were to fail (which is exceptionally unlikely), is backed by the US Treasury and the Federal Reserve. EDIT: Walking back to first principals, why can't the USD fail? Because you can print whatever adults in the room decide need to be printed to hold the economy up. You cannot do this with a deflationary digital asset, nor one tied to smart contracts.
The practical implication of solving large scale bank failures with mass currency printing is hyperinflation. You would only get your money back in nominal terms, but its purchasing power would be greatly reduced. See also: Weimar Germany and Zimbabwe.