This has happened to me more than once: We're in the chemistry/biology/genetics lab weighing out chemicals or pipetting reagents when someone says (half jokingly), "Why is all of this in grams and liters?? This is America!"
Well, little Timmy, this is why:
American colonists in the 1700's inherited and used the British Imperial System, which was a chaotic system of weights and measurements, while France developed and refined the metric system. In 1790 Secretary of State Thomas Jefferson was supportive of a decimal-based metric system, but feared that the US wouldn't be able to verify the metric unit of length without sending a costly delegation to France.
International tension also stifled US relations with France. Although France supported the American colonies in the Revolutionary War, the ratification of the Jay Treaty, which basically made France the third wheel in the American-British relationship, changed their opinion of the colonies. France became so hostile that in 1798, it ignored the US when inviting foreign dignitaries to Paris to learn about the metric system (ie. the US was not invited to the metric system party).
In 1821, Secretary of State John Quincy Adams was happy with the US Customary System and declared it sufficient, requiring no changes. Meanwhile, the metric system was gaining popularity overseas and by the end of the Civil War in 1865 most of Europe had adopted the system. Recognizing this, President Andrew Johnson in 1866 signed into law that the US "employ the weights and measures of the metric system in all contracts, dealings, or court proceedings."
In 1875 France decided to throw another metric system party and the US was invited this time. The Treaty of the Meter was signed by 17 nations, and the standards of measurement were set and everything was good to go. So after all of this drama why still hasn't the US transitioned yet? Because, little Timmy, Congress made the conversion voluntary. In 1971 the US Bureau of Standards published a report called "A Metric America" that recommended the US transition to the metric system within 10 years. Congress agreed, but made the switch voluntary and the excitement to go metric faded. International economics forced many companies to use both the metric system and the US Customary System, but for the most part, the US remains the only industrialized nation that hasn't adopted the metric system.
Changing gallons to liters and inches to meters is not as easy as it sounds. NASA has estimated that converting their charts, drawings, and documentation to the metric system would cost not only thousands of hours of work, but also $370 million. Even on a smaller scale change would be difficult. Every single sign on freeways and highways will have to be replaced with those that reflect speeds in kilometers per hour and we'll have to start thinking about whether $1.99/kg of potatoes is a good deal or not. I don't know about you, but I have no idea how many pounds are in a kilogram and I would need to bring a calculator every time I go to the store.
Alas, if only the transition were made mandatory Americans wouldn't have to learn two languages of measurement.
All information via How Stuff Works