Why won't USA adopt the metric system?

Americans don't like change when it is difficult. We tried changing to the metric system in the 70s and ended up with a load of people who don't know either the metric system or the imperialist system. My wife is one of them. It just isn't going to happen. You could ask the average American how much a liter is and won't even occur to them that they are drinking from a half liter bottle of Deer Park.