DoD can't afford the time or the money to teach programmers a new language every time they change projects. In the late 1960s, DoD was using nearly 450 programming languages. That number has been reduced to approximately 37, which is still too many. However, it's progress. Soon, few will disagree that Ada is the language of choice.
Every mandate comes complete with a waiver system.
We can't seem to birth one without the other. Waivers to the Ada mandate have traditionally been handled by the individual services, and the services have published the waiver rules. Obviously, the lifecycle cost justification is a major piece of the waiver request. We don't want people using the old excuses such as there aren't any bindings or tools. If there are compelling reasons and the agency can justify the lifecycle costs of using a language other than Ada, the services will probably accept it.