A good question but I think it had to do with reaching 3000fps with the powders of the day in a 30/06 case also I believe the Pedersen Device may have used a .276-.277" bullet in some of the proto-types and this may have influenced Winchester,
and just normal cartrige development may have been the answer.

I also have wondered why H&H went to .308" with their .300 H&H Magnum when the British were using the .311-.312" in the .303British cartridge and I do not think the British had any other ..308" cartridges.
Steve