That is a subject fraught with interesting plausibilities Chris....I mean,  why the stark absence of any reliable info?  I have searched, believe me.   In considering the average stature of these people, one might even conclude that their efficiency with the sling may have been reduced, comparatively, from a biomechanical standpoint.   This may have tarnished it's perceived effectiveness as an aerial bombardment weapon in comparison to the bow.  Also, if the bow was in use in Asia as early as some research has indicated.....then even the sling's usefulness as a personal hunting weapon would have been shortlived as well.