The DOD effectively can't "just mandate" things and it's because of politics (which is a superset of corruption and incompetence).

There's a very small number of major contractors and if you pick one of their APIs as the new standard, then you disadvantage all the others.

If you pick a greenfield project, then it will take a long time, and once you are done everybody will claim that they can't use it (see e.g. Ada; quick version they started working on a mandate for a programming language in 1975, mandated Ada in 1991 and by 1996 removed the Ada mandate).