(1) the infaliabilty of its public ledger (which means it is especially useful where trust is low and where trust is needed).
(2) The ease of making a transaction.*
*Aside: since the bitcoin transaction volume currently exceeds the limits it was specifically designed to handle, that means that transactions currently either cost a lot of money or take a long time to be confirmed. But those are issues that could be resolved either with lightning network (or other off-chain solutions), side-chains, forks (soft and hard), or even by a whole new variation of the blockchain concept. Although bitcoin transactions admittedly currently aren't cheap or fast doesn't mean blockchain necessarily will always be like that.
Except when they roll it back because people with more money than you demanded it after they got screwed somehow.
The downside is the US government's outlay is larger than it's income, but in practice that's less important than you might think.
The fundamental source of demand is the requirement that US Taxes must be paid in dollars. The utility of using dollars to trade with the US economy is simply a consequence of this fact.