Some misinformation here.

There were literally just as many Soldiers fighting and dying in the Pacific as Marines. For instance at the very first amphibious landings by the US in the war at Guadalcanal by the end involved more Army formations the Marines.

The initial landings were conducted by the 1st Marine Division, but rushed to the island as reinforcements was the 164th Infantry Regiment of the North Dakota National Guard, while following on in the next few months would be the XIVth Corps consisting of the 2nd Marine Division, and the 23rd and 25th Infantry Divisions of the US Army.

Then we can also look at the final major landing of the war at Okinawa. Of the 200k or so men involved in the landings as the 10th Army consisting of the IIIrd Amphibious Corps and the XXIV Corps. Commanded by Army General Simon Buckner Jr. (who was killed in action) it consisted of the 1st, 3rd, and 6th Marine Divisions, and the 7th, 27th, 77th, and 96th Infantry Divisions.

Part of the reason is that the USMC remained comparatively tiny throughout the war, topping out at 6 division sized formations in the field, while the US Army had 100's and could send them to every theater they were needed in. There was also the desire of the Navy Marine Corps team to fight the war they had planned, that being an island hopping campaign against Japan, which they had spent the 20's and 30's preparing for, so the institutional doctrine and know how was best applied in the Pacific not in a land war in Europe.

Then there is the fact that the US Army is plenty proficient on their own in opposed landings, and has done them for centuries just like the USMC, and that Marines did serve aboard ships in the ETO, so it depends on your definition of being "deployed" means.

Hope that helps!

Don't make the assumption the Army only fought in Europe, Italy, and North Africa. The Army was responsible for the southwest Pacific area, New Guinea, and the Philippines. The Army also fought on Peleliu and split the fighting with the Marines on Okinawa. The 32nd and 41st Infantry Divisions were among the first US troops to commence offensive operations in World War II. 22 Army infantry divisions were eventually used in the Pacific.

The Marine Corps has been under the control of the United States Navy since 1843, and hence is used to project the Navy's power from the sea onto land. The Marine Corps of WWII was not designed to take on lengthy land operations as the Army was, having a lack of organic heavy artillery (155 mm+), powerful antitank guns, and a shortage of organic reconnaissance troops.

The F-Series Marine Division, used in 1944, is here compared and contrasted to a US Army infantry division of 1944, and it is shown how the Marine division is better-suited to island fighting, which requires overwhelming short-range firepower, than the US Army division. The Army division is better-suited to a lengthy land battle because it has more antitank elements, reconnaissance troops, and long-range firepower.

The F-Series US Marine division had 24 light 75 mm and 24 105 mm howitzers, for a total of 48 pieces

The 1944 US Army infantry division had 36 105 mm and 12 155 mm heavy howitzers, again for a total of 48 pieces

The F-series Marine division has 243 flamethrowers in its organization; the Army division has none organically assigned to it.

The US Army infantry division had 36 57 mm antitank guns and well over 500 bazookas spread across the division. The F-Series Marine division had only 36 37 mm antitank guns and 172 bazookas.

The F-Series Marine division had three BARs per squad, as well as scrounged Thompson submachine guns and several flamethrowers when needed; the Army division on paper had only one BAR assigned per squad, and no submachine gun.

The US Army infantry division had an organic Cavalry Reconnaissance Troop with armored cars and jeeps; the Marine division had no organic recon elements.

Keep in mind that there were 68 US Army infantry divisions active during WWII, while there were only six Marine divisions. The Marine divisions could not be spared anywhere else, because at any one time, all six divisions were in some state of fighting, recuperating, or training for the next operation, and were rarely behind the lines, "off", in any sense. Completely retraining and re-equipping the Marines in the Army's doctrine of land fighting and then redeploying them to Europe would cost precious resources and above all, time.

You aren't completely wrong about Marines "fighting" in Europe, however. A few Marine planners assisted the Army in planning for Operation Overlord. There were small detachments of Marines aboard the battleships that bombarded the Normandy coast on D-Day and some Marines even expressed a willingness to go ashore and assist the Rangers at Pointe du Hoc if so needed.

Sources:

T/O 2-27 Cavalry Reconnaissance Troop, dated 15 July 1943

T/O 6-25 105mm Field Artillery Battalion, 27 September 1944

T/O 6-35 155mm Field Artillery Battalion, 27 September 1944

T/O 7-11 Infantry Regiment, 26 February 1944

T/O 7-17 Rifle Company, 15 July 1944

T/O F-100 F-Series US Marine Division, 5 May 1944

F-Series Marine Division Organization http://www.ww2gyrene.org/Division_Fseries.htm

ETO Order of Battle- DIVISIONS, Office of the Theater Historian, December 1945 http://www.history.army.mil/documents/eto-ob/etoob-toc.htm

US Navy Organization- An Overview http://www.navy.mil/navydata/organization/org-over.asp