The US didn't adopt the metric system because it was considered blasphemous since the French used it. I'm not joking. We got screwed by a bunch of demagogues.
The US uses customary measures. Imperial is the old UK system. It has some important differences like the size of a gallon - and perhaps most importantly if you like beer: the size of a pint.
There are a lot of situations in the US where using dual measurements, or just using metric is required, like food labeling. Using customary units is optional in many cases
