Legal Question in Criminal Law in California

I'm sorry to hear about your loss.

There's no way to tell why the D.A. rejected the case without more facts. Have you called them and asked? They may be willing to tell you.

Prosecutors reject cases for a variety of reasons. Usually it's because they don't believe they can prove the defendant's guilt beyond a reasonable doubt. But sometimes it's because they don't have jurisdiction over a particular crime, because the limitations period has expired, etc. And sometimes it's because they believe the defendant is innocent.

I also can't say whether the Marines will pursue charges. It seems quite plausible that the D.A. dropped the case so the military could pursue it, but I can't say whether that actually happened. That's a question of military law, which is a subject I don't know well. But there are plenty of lawyers who do, and some of them participate in this site. If you re-post your question under Military Law instead of Criminal Law, one of them might answer it.

Good luck.