From Ammoland:

The military has been used to indoctrinate service members for social engineering since at least WWII. Progressivism, as a philosophy, is anti-Second Amendment in its DNA. Within the elite, military people are expected to disdain, look down upon, and discourage the actual carry of weapons.
Candidate Trump may have underestimated the political correctness within the military brass. President Trump was never a bureaucrat. In Bureaucracies, there is a long known and practiced method of preventing superiors from implementing policies you disagree with. It is defiance by compliance.