In the early 1970s, the United States went off the gold standard and the previous system of fixed exchange rates between foreign currencies was abandoned.
Many restrictions were lifted on the flows of international capital, making it much easier for investors to purchase foreign securities.
The Reagan and Clinton Administrations in particular made de-regulation of capital movement.
Financial liberalization has been the most direct, and probably the single biggest, factor accounting for the growth of international investment flows over the past several decades.