In World War II the Americans Were the Only Armed Force to equip infantry with a semi-automatic rifle (M1) as the standard-issue weapon. Why was this?

Because infantry small-unit doctrines were different in different armies during WWII and arguably the US got it wrong. The reason why the Germans had the K98 which was essentially unchanged since the 19th century as standard issue is because the Germans realized that rifles delivered a low volume of fire and machine guns were the center-piece of an infantry squad. Each HMG (heavy machine gun) could deliver the same volume of fire as 20 riflemen. The Machine gun was what mattered in a squad and not the rifles. German riflemen were expected to defend, cover, spot for and support the machine gun instead of the other way around.

By 1944 the German army had the highest ratio of HMG: riflemen at the squad level of any combatant force in the war. During the battle of Normandy an US 29th Infantry Division rifle company had 2 machine guns for 193 men, while the German rifle companies opposing them had 15 machine guns for 142 men. The Germans had excellent machine guns, the MG42 was both combat effective and light enough to be carried by a single men and thus giving it mobility and firepower.

The US doctrine called upon the usage of riflemen, maneuvering, suppressing, flanking and overrunning the enemy, using M1s and BARs to maintain volume of fire to overcome the enemy. In the US army, rifles were what mattered thus the US issued semi-automatic and automatic rifles, however this proved to be an extremely problematic doctrine in the face of German machine gun fire and US infantry frequently had trouble defeating German squads without air and artillery support.

Source: Beyond the Beachhead: The 29th Infantry Division in Normandy By Joseph Balkoski