The main ideas shaping US imperialism were "Manifest destiny" and the "White man's burden". Manifest destiny refers to the 19th century belief that the United States were meant to expand across North America because of the particular excellence of its (white) people and (democratic) institutions. This lead to the Trail of Tears, among other atrocities. Once the US reached the Pacific Ocean, new lands came into view (Puerto Rico, the Phillipines). Manifest destiny became entwined with the idea that "civilized" nations had a moral obligation to colonize other peoples, which Rudyard Kipling summed up in his poem "White Man's Burden". The tensions in this "benevolent imperialism" were never fully resolved; the US invasion of Irak was launched under similar pretenses (the invasion was "for the good" of those invaded). The idea that other peoples have a right to self determination regardless of their level of development has not yet become accepted by the great powers, including the US.
Figuring that one out now.
Answer:
The biggest reasons the U.S. hasn't adopted the metric system are simply time and money. When the Industrial Revolution began in the country, expensive manufacturing plants became a main source of American jobs and consumer products.
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