If I had to speculate, I'd say most likely because there are zero Linux vendors willing to run the gauntlet of a DoD procurement process.
And agree with your point.
My question was more rhetorical :)
You know what DARPA should be doing? Instead of trying to make 'smart bullets' maybe they should consider that complexity and security of IT is a goddam problem and support products instead of just technology.
Someone to make a version of Linux, hardended/simplified it, and then did the same for a few other pieces (i.e. networking gear) and then made 'mil grade' versions of a bunch of things.
And then teach every single member of the military how to do basic stuff, i.e. 'know their computer'. Super basic 'MILINUX' skills, so that weapons systems techs could at least fathom what on earth is wrong with their thing and possibly fix it.
Same with mail.
There is no reason in 2018 that e-mail can't be durable and available for relatively low cost in some system.
Also consider now that there should be a lot more IT dudes in the forces.