Simply put, copies sold isn't exactly the same barometer it used to be simply because it's no longer the sole source of revenue. I forget the exact quote off the top of my head) Note that they very specifically did not say 'most copies ever sold.' In fact they haven't said a word about copies sold for almost any game this generation. Keep in mind that after Halo 5 launched MS's big thing they liked to quote was that it had the highest revenue of any Halo game ever. I'm not so sure lower than expected sales is the deterrent to a sequel you might think.
