McArthur kept the .30-06 because it was the standard cartridge of our machine guns, both infantry and air corps, and we had literlly billions of rounds of ammo in the war reserve.
We didn't have enough Garands to go around, and issued hundreds of thousands of 03 and 0A3 springfields, all in .30-06.. The .276 would have created a logistical nightmare early in the war when we were most vulnerable. McArthur made the right decision.
There'sw no question the .276 was a good casrtridge. Actually it may have been better than the .30-06 for military purposes. It just came along at exactly the wrong time.