The D-Day landings were the biggest undertaking of the US Army to that point in the war. The US Army was huge, but most of its divisions were totally inexperienced. Only a handful of divisions had taken part in the fighting in North Africa, and most of those were still engaged in Italy. To offset this inexperience, they had raised elite assault troops, such as paratroopers and rangers, reorganized their regular troops for assault landings, and brought experienced veteran divisions back from the Mediterranean. This gives an American commander a wide choice of forces, even though their equipment was standardized to maximize the benefits of American-style mass production.
Do you want to field fresh, eager troops, available in significant numbers, or do you want to field the less common desert veterans. Are you content with regular army troops, or do you want a small, elite strike force?