Regardless which case was used,the OP seems to wonder why the smaller case was used,versus the 30-06,which,because its bigger,must be "better"
Wasn't this a David Tubbs creation?
Mr Tubbs is a very successful shooter.He can have his own ideas about "better"
As I understand it,among his criteria,is the cartridge work in a 2.800 length magazine.
The long,high BC 6.5 bullets were the choice.
The case has to be short enough that the long skinny VLD ogive does not go into the case neck when the bullet is seated to 2.800
There is a 6.5 x308 called the 260 Rem. It does not make it that way.
I assume Mr Tubbs also wanted the following: For 1000 yd shooting,we just do not get "flat" trajectory. Folks use adjustable sights.
So long as velocity remains supersonic at 1000 yds for bullet stability,velocity is "enough"
More power = more recoil. Shooter fatigue. Folks shoot lighter kicking guns better.Especially for a lot of shots.
More powder=less barrel life. A 6.5 Creed barrel will far outlast a 6.5-06 barrel.
I assume the smaller bottle will have a more efficient, consistent ignition and burn,which means accuracy.
Disclaimer,I don't shoot competitively,and I don't own a 6.5 Creed.
Now,I'll reverse the question. For top level 1000 yd match shooting,why would you prefer the 30-06 case?
Note:If you look at the case dimensions 300 Savage vs 308,the head to shoulder dim is nearly identical.Which means,not a lot of difference in case capacity.Ths shoulder is steeper and the neck shorter on the 300 Savage.
For military use,a more sloped shoulder and longer neck are advantage,particularly in autoloaders.