The M1 and M14 were ideal weapons for skilled riflemen.If you can see your target you can shoot at it effectively.All the 5.56mm weapons,not so much.
I think that's the key phrase. The M-16 works well for an urbanized recruit base where many do not have much experience with firearms. Also, that soldier who may not be a highly skilled rifleman can carry a lot more 5.56 ammo.
Sure like that bigger slug the M-1 fires, personally, though.