From the History of the M1icon Carbine (Wikipedia)
"Prior to World War II, Army Ordnance received reports from various branches (infantry, armor, artillery, supply) that the full-size M1 rifle was unsuitable as issued for an increasing number of soldiers with specialized training (mortar crews, rangers, paratroopers, machine gun crews, radiomen, tankers, artillerymen, forward observers, signals troops, engineers, headquarters staff etc.) who did not use the service rifle as a primary arm."

"During prewar and early war field exercises, it was noticed that these troops, when issued the rifle [M1 Garand or 1903 Springfield], often found their individual weapon too heavy and cumbersome. In addition to impeding the soldier's mobility, a slung rifle would frequently catch on brush, bang the helmet, or tilt it over the eyes. Many soldiers found the rifle slid off the shoulder unless slung diagonally across the back, where it prevented the wearing of standard field packs and haversacks. Alternate weapons such as the M1911 pistol and M1917 revolver, while undeniably convenient, were often insufficiently accurate or powerful, while the Thompson submachine gun, though reliable, was heavy and limited in both practical accuracy and penetration at typical combat ranges."

From M1 Carbine: America’s Unlikely Warrior, by Garry James:
"The M1 Carbine was developed in response to a requirement for a light, handy rifle to be carried by clerks, cooks, linemen, machine gunners, mortarmen and the like—soldiers who would not normally be issued a handgun, but for whom the bulkier Garand might be inappropriate."