Perhaps metric units were superseded by "natural units", which are based upon the universal constants of nature, such as the speed of light in a vacuum, the gravitational constant and the Planck constant. The values of the needed constants were not determined in OTL until the early 20th century, which is rather late to establish a new system of units, but perhaps the Great War caused enough disruption that a window opened for such a change to happen. The metric system (and presumably France, where it originated) would need to be discredited enough to allow for this. Maybe in this timeline France was widely condemned as being responsible for the Great War, which was even worse than in OTL?