I don't need an explanation, I don't even want an explanation, we already have the definitive one (Hicks died because of Alien 3's development hell, now we're free to ignore that, so YOLO), I don't want this movie to care about continuity, disregard all that, make as little references as possible, ignore Alien 3 and R and and just give me a good ride with the things that made Alien and Aliens great: Great thrills, great creature, great characters (Ripley, Hicks, probably Bishop) and a great story on its own right.