The U.S. is indeed still number one by far in terms of number of millionaire adults (14 million; compared to 2.5 million for the runner-up, France); but the median U.S. adult is now 27th in terms of net worth.

The number one cause for this lag (though certainly not the only one): astronomical health care costs.

Median net worth per adult (US$ 000), from the 2014 Credit Suisse Global Wealth Report:

Australia________225
Belgium_________173
France__________141
United Kingdom__131
Japan___________113
Singapore_______109
Switzerland______107
Canada__________99
Taiwan__________65
Germany________54
United States____53
Chile___________17
China___________7
Brazil___________5
South Africa_____4
World__________4
Russia__________2
Indonesia_______2
India___________1

http://s3.documentcloud.org/documents/1312773/credit-suisse-global-wealth-report-2014-1.pdf