@Soulfox -- The United States made a serious effort to switch to the metric system in the 1970s, remember? We know that effort failed but what is confusing is why it did.
That is a hard question to answer and requires some study. However, there are some obvious reasons why the switch wasn't made. Think about road maps, for example. We understand it is 30 miles or so to the next city, but how far is that in kilometers? Speaking of kilometers, if the speed limit is 70 miles per hour, what does that work out to in kilometers?
And, while talking about cars, how about gas? How many liters will that car hold? For those who obsess over gas mileage, figuring out miles per gallon is easy enough, but what is the equivalent measurement in liters per gallon?
Those are a couple of example. The point is that converting to the metric system would be hard at first and then easy for subsequent generations. But which generation wants to bear the brunt of converting and having to rethink the way things have been measured for centuries?