As an American, I am familiar with both systems. They teach us metric in school, and we use metric exclusively in high school science classes. I memorized all those high school physics constants in metric (the acceleration caused by gravity is 9.81m/s^2). I can convert between miles and kilometers, feet and meters, and pounds and kilograms approximately in my head. The reason Americans use standard units is because all our recipes are in standard units, our major roads are spaced 1 mile apart, our speed limit signs are in MPH, our cars report speeds in MPH in large characters (k/h in smaller characters), our thermostats give temperature in Fahrenheit (digital ones have a non-default option to switch to Celsius), and food in the produce and deli departments are sold by either the pound or ounce. It would be very inconvenient to switch to metrics while nearly everything sold in stores and all our infrastructure is in American standard units. We would have to convert everything everyday. It's much easier to say the speed limit is 40 mph (rather than 64 k/h) since all the signs are in mph. Why should anyone go through the extra work of dividing by 5 and multiplying by 8 to get k/p when the signs are listed in mph and all your friends are used to mph? Metric would eventually be used by in everyday language if all of our equipment, speed limit signs, etc used it. When I go to Mexico, I use k/h because all the signs there use it. When in Rome and all. We don't have to use metric just because nearly everyone else in the world does. We're free to use whatever system we want.
Additionally, you may think a federal (national) law could get all our speed limit signs converted to k/p, but I believe each state (similar to a province) has authority over its own roads. Over the history of America, the federal government has become more and more powerful - assuming many powers of the states. States don't like the federal governments taking their rights and powers from them. If the U.S. government really wanted us to switch our speed limit signs to metric, they would have to pay the states lots of money. In the past, the U.S. government enforced a national speed limit of 55 mph by giving lots of money to states who kept all their roads speed limit at or under 55 mph.
Congress could force manufacturers to label products in only metric if there was a rich special interest lobbyist group that was really in favor of metric. A rich metric lobbyist group would be the most important thing to get our country to convert to metric. Large-scale public support would be second. Without either of those, congress doesn't have much interest to care. Many congress member are older people, and older people tend to have contempt for change. And perhaps more importantly, a large number of their constituents may see banning American standard units as interfering way too much in their day-to-day lives.
tl;dr - we use metric because all of our stuff and nearly everyone we know uses it. congress has no reason to enforce change.
