One area that hasn't been touched upon is social unrest.
All of the major periods of social unrest in the United ( specifically, the 1860s, the 1930s and the 1960s) were all time periods when there social unrest and a lack of economic opportunity worldwide. There was no reason for native born Americans to leave the United States and most did not. Without role models to make being an American expat "cool" or "unique" most Americans have decided to remain here at home.
Had there been social unrest, coupled with nations where Americans could have moved to and enjoyed the same standard of living or higher in the past, it's very likely that the percentage of American expats would be higher than it is today.
Jon Mixon, Murican..I am
Source: Quora Digest