Military personnel, at least career military personnel, are loyal to the country, not a political party. That is their oath. They do not abjure their duty simply because they do not agree with the sitting president’s policies.
(Note also that he doesn’t resign because he disagrees with the President. He wants them to resign.)
Being more loyal to the country, to the American people, than their political party is something the GOP doesn’t understand anymore. To them, it is all about their party being in power and obviously, they do not care if they deprive the military of leadership in a time of war in order to have that power.
Of course, they don’t care about the military at all (and they have blocked more since), unless it is for a photo-op. Sadly, people in the military are told that the GOP blocked all these bills because the Democratic Party added unrelated riders to them. They can’t point to any of the riders of course.
Occam’s Razor: Rather than simply voting to block the bills, there is a Democrat conspiracy to add unrelated riders so bad that the Republicans will hurt Veterans in order to stop the Democrats. Or the Republicans hate, hate, hate, hate all social safety net programs and including ones for Veterans?
They would far rather the majority of the DoD budget go to contractors than to military and former military personnel.