It boils down to numbers, the realities of the force, and need or lack thereof.

For reference about 16,000,000 Americans served in uniform during WW2. 11m or so were in the Army/Army Air Forces. But just 650k were ever in the Marine Corps. The Army fielded a grand total of 91 Divisions, the Marine Corps just 6, and of course numerous independent, garrison, and support elements were part of each.

So there were a hell of a lot more soldiers to fight a global war than there were marines so send around. All 6 Marine divisions wound up in the Pacific, along with 22 Army divisions, though several of those were just arriving in the summer of 1945 and several more on deck that had previously served in Europe. So the USMC wasnt even the majority of the fighting force in the PTO. Though they did form the majority of the 2 Amphibious Corps(I/III and V) which alternated serving as the landing force in support of the Fast Carrier Force as it fought its way across the central Pacific. While the fighting in places like New Britain, New Caledonia, and the Philippines were primarily Army campaigns, while even some of the most famous battles like Okinawa saw more Army soldiers landed than Marines.

But why was the USMC so small? Well it comes down to mission, and budgetary battles in the interwar. The nation didnt need 2 armies, and the role of Marines, and which the USMC carved out, was as the small expeditionary force. It didnt need to be big, just enough to fight in the Imperial police actions in the Caribbean and Central America, and to seize and defend forward bases for the Navy.

So if the USMC is going to be joined at the hip to the Navy and had spent the past 20 years thinking about how to support a cartwheeling naval offensive, it is a no brainer that they will be concentrated where they would be doing the most cooperation in the theater dominated by the naval war.