Because:

1. Economic Power

The main cause why a country's currency can dominate, of course number one is its economic strength. In the past, the basis of his assessment was how much gold was available, The greater the Gross Domestic Product (GDP) of a country and the volume of cross-border transactions, the greater the demand for its currency.

2. Military Superpower Countries

Military strength, both in terms of personnel, weaponry and intelligence of the United States is difficult to find a match. Throughout the history of mankind, especially in the era of colonialism and world war, a series of events in which a stronger currency was accompanied by a dominant military force was not just one or two.

3. Prestige of Foreign Diplomacy

Every country tends to conduct trade and financial transactions with other countries with whom they have good relations. This is one of the strongest reasons why the dollar is the world's currency. The strength of US diplomacy outperforms other countries, even compared to those who have the economic and military power almost able to compete. This is because the prestige of diplomacy is not about how many countries are "scared", but rather how capable the diplomat team lobbied, made friends, and expanded the network.

4. Stability and credibility, accepted everywhere

If you want to go shopping at the mall, then of course you will make sure that in the wallet there is money that applies legally (not counterfeit money) or a debit / credit card from a bona fide bank, can be accepted at many merchants. As well as, the perpetrators of transactions between countries will ensure that the currency used is credible and will be accepted by the counterparty. Many countries make the US dollar the main currency of its foreign exchange reserves for this reason.