Because it's the global reserve currency it generally means:
1. There's more people around the world with USD who are interested in putting it to work.
2. The US government can easily print and borrow money denominated in USD, which leads to more USD floating around, which ends up in the hands of people from #1.
I don’t have any references handy, but I suppose it’s a combination of the U.S. being a top destination for investment (because of the currency, risk-level, innovation and productivity), plus high demand for labor.
1. There's more people around the world with USD who are interested in putting it to work.
2. The US government can easily print and borrow money denominated in USD, which leads to more USD floating around, which ends up in the hands of people from #1.