From Academic Kids
The U.S. is the only country that has a government-specific top-level domain in addition to its ccTLD. Other countries typically use a second-level domain for this purpose, e.g., .gov.uk for the United Kingdom, .gc.ca for Canada, .go.jp for Japan, .govt.nz for New Zealand and .gov.it for Italy. Since the United States controls the .gov Top Level Domain, it would be impossible for another country to create a domain ending in .gov, for example .jp.gov.
The usage of .gov as a gTLD controlled only by the U.S. is controversial, as some people believe this to be an example of arrogance by the U.S. - such views hold that usage of .fed.us or a new second level domain of .gov.us would be more suitable.