I think the reason most members of the military are Republicans is accounted for primarily by group psychology. They join because that’s what their conservative social group says they should do, and when in the conservative social group in the majority keeps them within that same sphere. I think that is the biggest reason for the discrepancy. Basically peer pressure. I think the reason conservative ideology is generally associated with the military, and the reason that base already exists, is conservatives generally have two ideals that lend towards military service. Those being the concept of tradition and the concept of authority. Tradition compels them to both act in a traditional manner, and do support traditional things. A nation-state is a construct that depends on tradition, that is current acknowledgement of both it’s existence and history, and by extension the nations “values”. The tradition must be preserved. The concept of authority causes both a willingness, possible even a desire, to both follow and have authority. Authority also couples with other factors and produces the idea that the nation is the ultimate authority and must be preserved.

That’s probably a bit of an esoteric explanation. I hope it makes sense.