Issues of game design, I'm guessing. From what I gathered playing a bunch of instant action missions, most missions have parameters for "harrassment"-class vehicles. Fast, fragile units, basically. It just so happens they don't actually work in airless environments, fluff-wise. So the designers had the choice of either keeping missions consistent across environments and break physics in a way a lot of people wouldn't really notice, or risk airless missions being balanced differently (either easier due to the lack of harrassers, or harder due to having to increase the numbers of other units).
Clearly, they went for consistency. Whether they did it for the sake of game design, laziness, or just a lack of priorities, that's anyone's guess. Personally, I just think that a relatively minor physics inconsistency with vehicles that go up in flames if you so much as look at them funny, in a game about big stompy robots, is not a big deal. Now, if we were playing as the VTOLs and hovers in airless moons, then I'd be banging a much louder drum.
Really, I'd just rather have way fewer vacuum missions to begin with.