And then there were 18th century politics:
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The first practical analysis of this provision fell to George Washington's Secretary of State, Thomas Jefferson, in 1790. Jefferson endorsed a decimal system of measurement but, when presented with the basic principles of the decimal-based metric system, felt reluctant to steer his nation in that direction. He feared that the U.S. wouldn't be able to verify the metric unit of length without sending a costly delegation to France.
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The evolving political situation didn't help matters. Although France supported the American colonies during the Revolutionary War, it became hostile to the U.S. after Jay's Treaty was ratified in 1795
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In addition, there was concern among American statesmen that the French commitment to the metric system might falter in the aftermath of Napoleon Bonaparte's ill-fated reign during the early 19th century.
Then came the 19th century;
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Over time, however, the metric system gained traction. By the time the American Civil War ended in 1865, most of Europe had adopted the decimal-based measuring system, and the U.S. could no longer ignore it. In 1866, an act of Congress, signed into law by President Andrew Johnson, made it "lawful throughout the United States of America to employ the weights and measures of the metric system in all contracts, dealings or court proceedings."
http://science.howstuffworks.com/why-us ... system.htm
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