I mean, it's obvious why, just military dominance.
There's a lot of truth in that. London UK became the financial center of the world thanks in large part to the British Empire. And with American geopolitical predominance, history has repeated: trillions upon trillions of greenback-equivalent just keep knocking of the door of New York to be let into the American capital markets.
And here's a fun fact: when the sun sets on the geopolitically-predominant power, two aftereffects persist: 1) world-class standing in the capital markets; 2) a bloated military budget.
The shift of the military and financial capital of the world from the USA to China+Asia won't be complete until 2032.95 (thus isn't relevant to our current discussion):