To put it simply, there werent that many Marines and the Army was just as capable. By 1944 the Army had conducted even more landings during the war than the Marines and had developed the same expertise from North Africa, Sicily, and Italy along with the Pacific.

Even in the Pacific the Army made up a major amount of the troops involved. For instance of the 180k troops to land on Okinawa, 102k were soldiers. And the fact is tha even with the expansion the Marine Corps never grew all that large, topping out at 6 Divisions which formed 2 different Corps both of whom operated only in the Pacific.

It should also be noted that at least on Omaha the first wave was only made up in a small portion by Rangers, the larger part of the initial assault wave was from the 116th and 16th Regimental Combat Teams from the 29th and 1st Infantry Divisions.