It's a fantasy game in space. Don't expect scientific explanations to anything.
At this point, I don't expect any explanations to anything. Just cool set-pieces.
And man, wouldn't it be cool to fight the Fallen or the Hive in amongst the ruins of an old Apollo voyage on the Moon?
I don't want to fight near anything related to historical missions. I just want to be able to go see them. Or find them hidden away somewhere. Maybe Curiosity's mast-cam poking out of a sand dune, or something. The last thing I want is to see Fallen/Hive rounds ripping through the gold foil on the Apollo LM's descent stage. That'd break my heart.
EDIT: Also the explanations thing, I don't really need an explanation. "Rule of cool" would be just fine with me, but I'd still be rubbed the wrong way by the space station above the Moon never moving, or the Earth being so very nearby in the sky, or Phobos being so close to Mars, or being able to see Mars as so large from the surface of the Moon, or...