I'm not gonna pretend to know anything about game development, but I remember reading something about the development of the Mantis in Halo 4, and how it was different from any other vehicle in previous Halo games. The game engine treats the Mantis as a biped (think Spartan or Elite), rather than as a "true" vehicle. Thus why it can't ever fall over (unless it's dead) or be flipped. Maybe this has something to do with it? Since the game engine doesn't truly recognize it as a vehicle so much as it does a character model.