The demand for a key currency, like the USD - in contrast with other national currencies - is based not only on the domestic economy, but the global word economy. As the USA is viewed as a superpower and a global leader in economy, accordingly the US currency is generally viewed as a stable monetary asset which can be used as a global currency in international trade, or a clearing unit in international transfers and a value safeguard.
Some other currencies are tied to the value of the USD as well, which makes global trade much more calculable. As a result of this collective perception, the demand does not depend that much on the US economy, but the global economy and other global processes. This is obviously beneficial for the US economy and government as well, as it has much more wiggle room in becoming burdened with debts, while able to keep the value of its currency.