The British before WWI had decided to go to the 7mm and developed a new rifle, the pattern 14 Enfield for it. Their decision might have been greatly influenced by the fact that the South African Boers used the 7mm Mauser with great effect against them around the turn of the century.
When WWI began, the decision was made to stick with the .303 to prevent supply problems. I think I have read that Churchill himself made the decision.
The pattern 14 was chambered for .303 and contracts made to have it built in America. When the U.S. got involved in the war, we could not provide enough 03 Springfields and the decision was made to modify the Enfield for the 30-06, thus the 1917 Enfield.
I was just pinging Archy to chime in with his links and much greater knowledge on the M1 .30/.270 subject.