you also forgot capacity. same reason why they went with 5.56 over 7.62. you can carry more smaller bullets at the same weight.
The M16's 5.56mm bullet was deemed to be less lethal. Thus it was more desirable for the effect of draining enemy personnel because of the resources needed to keep a wounded man alive on the battlefield, versus simply documenting a KIA.
I stated the NATO requirements, but failed to mention the Vietnam scenario. We "needed" a smaller battle rifle since our allies in 'Nam were of a smaller stature and thus unable to master the M14 due to it's overall size and recoil. But there is no doubt that the adoption of the M16 was a financial boondoggle for the entire military industrial complex. Every bit as bad as the conversion from 45 to 9mm. I'd like to see a return to the M14 as well. For awhile the 6.8mm looked to be a step in the right direction, but I think that has pretty much died away now.