My guess is NIH--Not Invented Here. Also it was used by the losing side. Why use a round used by losers? We ended up with what is arguably the most powerful cartridge designed for use in a main battle rifle, the 30-06.
Let me see, the first world war was from 1914 to 1918. The 30-03 and then the 30-06 were created in 1903 and 1906 respectively. I think you are confused.
The 7X57 was arguably the premier martial cartridge at the turn of the last century and the Mauser was arguably the premier martial rifle. There's a good reason they were copied and the US did have to pay royalties to Pauser for a while because the 1903 action was essentially a direct copy of the Mauser 98 action. I think the paint was different.
