Because Nixon and Kissinger made a deal with the Saudis to conduct all their oil transactions in dollars in exchange for military protection.
The dollar was probably the most desirable currency for international transactions before that though. The USA was the largest economy by far, and USD was convertible to gold at a fixed rate.
I remember being a kid and hearing about a family member's workplace instituting a rule against discussing pay after some people got upset they were making less than their coworkers and started advocating for higher pay.
Neither I nor anyone in my immediate family was a leftist, but it seemed really obvious the primary reason for the rule was that management didn't want people to have reasons to demand higher pay.