America is the biggest stakeholder of this world because they have huge nuclear warheads and have enough military, political and economic status. Also freedom of speech made this country great also. i think the biggest reason is education, because in world top ten universities list 8 are from USA. 
I guess economy is the biggest reason. Their GDP is close to $20 trillion and therefore they can afford to spend as much as $600 billion every year for defense purposes. Another important thing to remember here is that the USD is the reserve currency of the world. Therefore the Americans can get long term loans at very low interest rates.