It could be that only the Sphere had the abilty to travel in time. But they needed a a Cube to carry the Sphere because it may have not had the long range abilty the Cube had. Also, the Sphere was very weak as we saw, it destroyed with just 3? Quantum torpedo's. So the Cube was needed to get it to Earth, once near there it could have gone back in time. I think that is a reasonable explanation.