Because none of the alternatives are large enough. If you are a central bank, and you need to deposit $100bn...you realise that you can't walk into your local b

Because none of the alternatives are large enough. If you are a central bank, and you need to deposit $100bn...you realise that you can't walk into your local branch of Chase and ask if they will take a cheque. If you give it to a bank, you are now an equity investor (as Middle East central banks found out in the 80s). If you invest in a non-USD currency, you are exposed to that currency in size and you probably can't sell (as Commonwealth central banks found out in the 60/70s). USTs are the only market where you can trade tens of billions easily. That is it. There are no alternatives. China and Russia have both tried to exit, and all they have managed to do is buy a few billion of gold. The ECB is trying to create an alternative but EU financial markets are microscopic because their banking system is so large/insolvent. There are also economic reasons, the US is the largest economy in the world, almost all trade is priced in USD so exporters/importers will earn/sell in USD which will usually be key to other areas of central bank policy. But yes, the main issue is quite fundamental: no other currency is large enough.