The United States, Liberia and Myanmar (Burma) use the same system. Metric is easier to use, less hold over conversions from earlier systems, but Americans can be slightly stubborn. Metric was just a term when I was in school. I deal with it as I work on a lot of cars. What frosts my buns is cars and trucks from the late 1980's through the 90's that use both systems. Mechanical bolts are S.A.E. and body hardware is metric. I just want to scream "Make up your mind" at times.
I can see using up stock on hand, but that doesn't take THAT long. 1970 was the last year Chrysler used LH lug studs. A few early '71 models had them as stock was used up, but they didn't keep ordering them.