This is the exact explanation.
I have submitted some mechanical prints with dimensions in mm to our machinists, and they looked at me like I was just crazy. They literally said "This is America!", like imperial units are the shit.
The engineering side of the American space industry works in imperial units exclusively. The science side of course follows SI convention. Honestly, I'm surprised that it's usually not too much of an issue, Mars Polar Lander and other notables aside.
Of course, with modern 3D CAD, all I need to do is click about 4 or 5 times, and I can get you the moments of inertia for an entire spacecraft spit out in any system of units you fancy, in any coordinate frame you fancy.
What's amazing is that the rest of the world, as far as machine tools goes, are still on Imperial.
Francesca is a bleedingly italian motorcycle. The guys I talk to to keep her running are either English or German. And you know what? They all use 1/4" or 1/2" drive wrenches. Sure, the actual sockets are something civilized like 10mm, but they aren't on a 12.7mm end wrench. Just today an Australian told a Briton to use a 24" 1/2" socket extension to fit a german part onto an Italian motorcycle.
- What's amazing is that the rest of the world, as far as machine tools goes, are still on Imperial.
Some quick research points to Britain's early dominance in the Industrial Revolution, combined with their metrication process not seeing official adoption until the mid 1960's. Apparently they still use imperial units quite a bit.
Anyway, it's almost time for a pint (WINKWINK) here in CDT.