Why would the French military use an engine designed in 1930 in their military vehicles through the 1980's? Isn't there any other engine that could do the job as well at comparable cost?
Some answers might be that the engines are simple, and therefore easy to maintain in the field in poor conditions. The parts count is lower than for overhead valve engines (no rocker arms, bushings, adjusting screws, pedestals, bolts, bushings, rocker covers, and no pushrods).
The engineering costs may (!) even have been amortized by now.
The French military no longer uses them; and they are now entering the US market. Here are some photos and observations of a bare block. The dark blue material on the unpainted surfaces is a French version of Cosmoline. The blocks are new, in excellent condition, and of course, once they are gone....they're gone.