Was it simply because of a lack of manpower and supplies to send to the front?
And in the case I did miss anything earlier, I'm talking about the European Theatre.
The answer here is partly landing craft and shipping and partly grand strategy.
After entering the war, the US High Command prepared a 'Germany First' strategy which essentially meant (to the US Army at least) that their top priority was to amass an Allied Army big enough to defeat the chunk of the German army that could be spared from the Russian front and get it into Northern Europe as quickly as possible. The U.S. Army did not want to fight with a small force sent in a Corps at a time, and fight by attrition, they wanted a force large enough to bring on a German collapse before launching the decisive operation.
North Africa and to some extend Sicily and Italy were intended as a target of opportunity in spots where the Axis couldn't easily deploy their whole force. During that whole time, the main effort strategically was the build up in Britain for the cross channel invasion.
The United States had only started seriously planning for war in June 1940 after the fall of France, and were not very prepared even at the end of 1941 before Pearl Harbor. The US entered the war in December 1941, which is only 13 months until 1943, and only 30 months until the Normandy invasion in June 1944.
In order to mount an army group sized invasion in Normandy, the Allies would need about 4000 landing craft, including about 400 large LSTs (Landing Ships, Tank). By the end of 1942, the US had produced only 23 LSTs and only a few hundred landing craft. Behind the Landing craft, the United States needed to produce enough shipping to support landings and large forces in combat. Liberty ships began production before Pearl Harbor, but only a tiny fraction of the 2700 built for the war had been completed by the end of 1942.
Although much else had to be done in raising and training the Army itself from a small peacetime force, and producing enough equipment and munitions to support it, the bottleneck in the European theatre that restricted operations was landing craft and shipping and would be until the last month of 1944 when actual manpower shortages began to appear.