It wasn't until the Metric Conversion Act of 1975 that the US and Canada started to switch. Canada got KPH but still sold gas by the Imperial gallon, and the US got two liter Coke bottles; but that was as far as it got.
Because the US is the only country in the world that still uses the Imperial system (courtesy of the British Empire and King Henry I of England, who ruled England from 1100 to 1135 with a twelve inch long foot). An inch is the width of a man's thumb and we all know from the Bible about cubits and spans. Not very accurate for space travel or any other scientific goal however.
It is because of the fact the entire planet uses Metric that all of our imports are metric sizes, our cars have metric fasteners (but with bores and strokes designed in inches). And it is why eventually even a hillbilly from the Ozarks will eventually start talking about klicks for distances (thanks to the US military using metric), and the NRA pushing NATO round assault riffles for hunting rabbits.