The US Dollar is the primary currency in which trade is conducted in much of the world. The US banking and financial system has much of the most powerful entities in the world relying on it for their future financial gains.

China, by comparison is a relatively more fragile economy with much of the growth there coming from manufacturing and real estate , both of which are sectors that are starting to struggle because, either there's pressure to move manufacturing away from external forces or internal force in China are clamping down on real estate speculation to free up the capital allocation crisis there where people would rather put their money on brick and concrete rather than back scientific innovation or commercial enterprise.

In short, the US can print $4-$5 Trillion dollars and get away with it while China simply does not enjoy the same luxury.