The first M1 Garand was developed in a .270 cartridge and might have been a better round for WWII, but Mc Arthur, Army Chief of Staff vetoed the idea as millions of 30.06 rounds were still in stores after WWI.
The cartridge follies continue to these days.
Actually, the M1 Garand was part of a competition for both a new service rifle AND a new service rifle round.
The Army wanted an autoloader, and both Garand and John Pedersen (Remington's chief designer for years ) submitted rifles for consideration. They were originally both submitted in .276 caliber (designed by Pederson), and then also in .30 cal when the Army decided it wanted to stick to the .30-06 cartridge. The Garand rifle won the trials, and for good reason. Garand's was the better design. Pedersen developed a good cartridge, though. The Ordnance boards actually preferred the .276 Pederson round*, but were overruled, in part because the final decision was made in 1932 - the worst part of the Great Depression in the US, when the government was trying to cut as many costs/expenses as possible.
Aside from the "we've got lots of .30-06 ammo, and we don't want to complicate supply issues by having two different rounds at the platoon level", the .276 P. got a bad rap because Pederson goofed a little in making his prototype rifle - it required the cases of the .276 round to be wax-lubed externally in order to provide reliable extraction - while the same round in the Garand prototype did NOT require case lubing at all.
While what I have read indicates that the .30-06 in Pederson's rifle also required case lube, while the Garand's ability to operate both rounds unlubed worked in its favor.
.276 Pedersen Rifle
* The .276P, despite having a lighter bullet, actually had better long-range ballistics than the .30-06 - its flight path was flatter to between 800 and 900 yards, and had higher retained energy between 400 and 800 yards (past 800 yds the .30-06 does better, but that was mainly useful for snipers, not common infantry). Additionally, with a ~1/2" shorter case of smaller diameter than the .30-06, and the ability to hold 10 rounds in the Garand (vs the 8 allowed by the .30-06), more rounds of ammo could be carried for the same weight and volume in the can and in the Garand's magazine.
for those who think that the .280 british and the .276 pedersen were pipsqueak cartridges, in comparison to the .30-06 or the .303 british ...