The U.S. is no longer qualified to host a World's Fair as it is no longer a member of the Bureau of International Exhibitions. Apparently, the US revoked its membership in this group during the 2005 World's Fair in Japan, in which the government was told it could not sponser any exhibits. According to all reports I could find, only private business were allowed to sponser and the US government took offense at this, and pulled the membership, making us inelible to host the World's Fair any longer..
This isn't exactly true. The United States originally joined the BIE in 1969, but hosted several BIE-sanction world's fairs before then.
It is possible now for it to do so again without BIE membership, but it does make it more difficult politically with the BIE having 100+ member nations.
The issue of BIE membership (which lapsed under the George W. Bush administration) is unrelated to the issue of US Pavilion funding (which was written into law before Expo '92 in Seville, Spain).
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