Because women are under represented in the military.
So? Is that a problem? Is the goal of the military to make sure that women are properly represented or to be effective on the battlefield and win wars?
You're missing the point, 350 Driver. It is not representing women for the sake of representation, but because sidelining 50% of your population for no good reason undermines your capability. The MoD isn't saying with this advert that only women need apply, it is saying that it has identified a gap in its recruitment that it is trying to address.