The attractiveness of the United States as an investment destination during the technology boom of the 1990s and the depth and sophistication of the country's financial markets (which, among other things, have allowed households easy access to housing wealth) have certainly been important. Another factor is the special international status of the U.S. dollar.
Because the dollar is the leading international reserve currency, and because some emergingmarket countries use the dollar as a reference point when managing the values of their own currencies, the saving flowing out of the developing world has been directed relatively more into dollardenominated assets, such as U.S. Treasury securities.