When I was growing up in the 1980s and 1990s, in the US, we learned both systems. Rulers had inches and centimeters. All packaging had both units. All highways

When I was growing up in the 1980s and 1990s, in the US, we learned both systems. Rulers had inches and centimeters. All packaging had both units. All highways signs had both units. I don't know why we "regressed," other than to say that when you live with both units, you realize that some metric units are silly, and some metric units make a lot more sense. More specifically, requiring that all units be merely powers of 10 of each other makes it very hard to use the most appropriate unit for the scale you need. (Don't believe me? Why do we have 24 hours in a day? 7 days in a week? 356.25 days in a year?) IE, it's a lot easier to estimate short distances using feet or millimeters; but kilometers makes a lot of sense for long distances. Understanding "the weather" makes more sense in Fahrenheit, but general science makes more sense in Celsius.