The reason he isn’t charged with the death of the unborn baby is precisely because it IS being tried under the Uniform Code of Military Justice (UCMJ), the legal code for the military.
As you may recall, shortly after our current President took office he issued an Executive Order that canceled or negated all previous Executive Orders unless specifically addressed. This is pretty common. One of the Executive Orders that was negated was an Executive Order issued by George W. Bush dated April 18, 2007. In that Order, President Bush added language to Article 119 of the UCMJ (the article that covers manslaughter), adding this change:
Note: For reference, Article 118 covers murder, 119 covers manslaughter, 80 covers attempts (at any crime)
(f) By inserting the new paragraph 44a:
"44a. Article 119a – Death or injury of an unborn child
a. Text.
(a)(1) Any person subject to this chapter who engages in conduct that violates any of the provisions of law listed in subsection (b) and thereby causes the death of, or bodily injury (as defined in section 1365 of title 18 to, a child who is in utero at the time the conduct takes place, is guilty of a separate offense under this section and shall, upon conviction, be punished by such punishment, other than death, as a court-martial may direct, which shall be consistent with the punishments prescribed by the President for that conduct had that injury or death occurred to the unborn child’s mother.
(2) An offense under this section does not require proof that –
(i) the person engaging in the conduct had knowledge or should have had knowledge that the victim of the underlying offense was pregnant; or
(ii) the accused intended to cause the death of, or bodily injury to, the unborn child.
(3) If the person engaging in the conduct thereby intentionally kills or attempts to kill the unborn child, that person shall, instead of being punished under paragraph (1), be punished as provided under sections 880, 918, and 919(a) of this title (articles 80, 118, and 119(a))** for intentionally killing** or attempting to kill a human being.
(4) Notwithstanding any other provision of law, the death penalty shall not be imposed for an offense under this section.
(b) The provisions referred to in subsection (a) are sections 918, 919(a), 919(b)(2), 920(a), 922, 924, 926, and 928 of this title (articles 118, 119(a), 119(b)(2), 120(a), 122, 124, 126, and 128).
(c) Nothing in this section shall be construed to permit the prosecution
(1) of any person authorized by state or federal law to perform abortions for conduct relating to an abortion for which the consent of the pregnant woman, or a person authorized by law to act on her behalf, has been obtained or for which such consent is implied by law;
(2) of any person for any medical treatment of the pregnant woman or her unborn child; or
(3) of any woman with respect to her unborn child.
(d) As used in this section, the term “unborn child” means a child in utero, and the term “child in utero” or “child who is in utero” means a member of the species homo sapiens, at any stage of development, who is carried in the womb.
There is more to it; it goes on to define things like an unborn baby killed by an arsonist would be covered under manslaughter or murder articles, etc. but this is the gist of it. This particular Executive Order caused a lot of angst among progressives because it contains a provision that applies these laws (possibly) to medical care of the mother and baby - in other words, it made an intentional abortion equal to murder if performed by a military doctor.
Now that this Executive Order is history in the current administration, the military prosecutor has no article with which to charge the accused with murder as regards the unborni. Best they might be able to do is an article 119 manslaughter charge, and that would not merit a death penalty. So in reality, that would have to be prosecuted under other federal or state law.
Given the high profile and gravity of this case, I can’t blame the prosecutor for not wanting to get bogged down in a fight over “what is unborn?” in one case of manslaughter when you have 13 other clear cut cases of murder that will get as much penalty or more. Recall also that the Commander in Chief is already on record as saying the decision of whether a fetus is human life is “above his pay grade.” If he can’t make a decision like that, it’s doubtful the Sec. of the Army or Sec. of Defense will be able to make it either - both of them are below his pay grade.