So for the same reason they used the 5.56 for 50years?
Not hardly. There is no comparison. A 140 grain 6.5 will shoot flatter, buck the wind better and retain velocity better than a .30 168 grain that leaves at the same speed.
1000 yard benchrest competition is dominated by various 6.5mm and 6mm fast twist cartridges because of their ballistic advantage.
Here’s a good article on the two cartridges.
https://www.shootingillustrated.com/articles/2017/11/3/308-win-vs-65-creedmoor-which-is-better/