One big advantage of using Linux or Unix is that you can very easily trim out of the OS what you do NOT need to run the system being developed and still have a tight secure operating system. Many manufacturers of machinery have been doing this for over a decade for their embedded systems, it makes sense to do the same for the military systems.

About fifteen years ago I worked in the Australian Department of Defence and we had a project that needed new software to do all sorts of trick things. The military officer in charge let his senior tech head loose and it was almost finished in Linux when the military department head changed and the new guy made them restart the project using Win NT as he understood Win NT but nor Linux. The change added three years to the project, much of it spent making NT more secure. I just hope this doesn't happen with this project.
edit to fix typo