Can you explain why the United States uses Fahrenheit instead of Celsius?
The answer to the your question is in Sir Isaac Newton's first law of motion, which is more commonly known as inertia. That is, that when something gets going it is hard to stop it or change its direction. It would be a really good thing if we just bit the bullet and made the change to the metric system or, more properly, to International Units. Even NASA just admitted to losing a Mars probe due to an error in converting from one type of unit to the other. We waste a lot of money using inefficient English units. I certainly do, since I have to work in both science and industry, I have to do a lot of converting back and forth.
The conversion to International Units is not without problems though. For example, an average machinist has thousands of dollars in measurement tools, almost always personally owned. They will have to replace every one of them to keep working in a metric world. Most machinists are resisting that change even though metric units are far easier for them to use.
The United States only has 3% of the world's population, but I am not being nationalistic when I say that the US is the dominant force in the world economy. That being the case, there is little force on the US economy to change and meet the requirements of other countries. In some ways our resistance to change drags other countries also, as some countries use English units to be able to trade with us. The change is happening, although at a slow pace. A few years ago, I had a car with a metric engine, but the body parts like nuts and bolts were all English. My current car is all metric.
Statesman of science Thomas Jefferson led America's first strong effort to switch to a decimalized overall measurement system. The effort failed, thanks to the very same social inertia Brian alludes to in the text above. But the envisioned system much resembled the modern metric system - and in principle it was the modern system's equivalent, in that it was base-10, was linked to universal physical standards, and was intended in a combined scientific and commercial spirit.
Answer 1 - Brian Kross, Chief Detector Engineer (Other answers by Brian Kross)
Answer 2 - Steven T. Corneliussen, Special Assistant for Science Communication, Accelerator Division (Other answers by Steven T. Corneliussen)