When you asked this you probably had no idea how complex (and in my opinion interesting) the answer to this question really is. The reasons touch on the development of technology, national pride, inter-service rivalry, prewar military contingency planning, strategic views and disagreements on how to fight the war within the American command that spilled over into disagreements with the British command, the specific threats offered by the Japanese and German militaries, and oh, the development of military aviation. Specifically the explosive development of strategic bombing capability, doctrine on air combat, and naval aviation.

Before the war the American command had a contingency plan called Orange that was obsolete, and a series of plans under the Rainbow name were developed as contingencies for a war in the Pacific. Rainbow-5 for instance outlined a joint Army/Navy mission to defend Hawaii from Japanese attack. After December 7, 1941 things changed, and The US had to make some strategic decisions on how to handle both Germany and Japan. While the biggest decision, pushed by the British was the Europe First decision to focus on Germany first then take out Japan plans also had to be made to defend the parts of the Pacific the Japanese had not yet taken (particularly Australia and Hawaii) and how to take Japan out when their turn came.

Europe first is often presented as an obvious and widely agreed on solution to the overriding strategic concern of fighting a two front war. It was in Britain, it was definitely not in the US. London was only a little over 200 miles from occupied Paris, and more importantly the English Channel is only 21 miles at it's most narrow point. Additionally, the British had already faced the most harrowing parts of the Battle of Britain in 1940, long before the US entered the war, and were under threat from strategic bombing and later rocket attacks for most of the war. Europe first wasn't just an efficient strategic plan in London, it was a response to an existential threat. Britain would defend Britain first, then worry about Asian colonies and Commonwealth allies like India and Australia.

The Americans had a different issue: Pearl Harbor. The Germans had attacked London itself, repeatedly through the air, the Japanese were a far distant problem to the Brits. The Americans had been hit in Pearl Harbor though, and taken heavy casualties. The Japanese were the more present threat, the Germans distant. Additionally, while all Americans took Pearl Harbor personally and wanted revenge against the Japanese, no one took it more personally than the US Navy who had taken the brunt of the damages in loss of equipment (battleships), men, and pride. The Navy was very focused on Japan, and the Army (George Marshall) was more in agreement with the British that it made more strategic sense to take out Germany first, because Germany was perceived to be the more dangerous opponent. There was a split within the Army ranks too though. Marshall was a proponent of Europe First, but the other most prominent Army officer was Gen. Douglas MacArthur, and MacArthur had lived in the Philippines for an extended period, ran their military, and was in command when the Japanese attacked there. MacArthur was obsessed with taking the Philippines back, and while controversial he was the best known and most popular officer in the military at the start of WW2.

The US adopted the Europe first strategy, but without enthusiasm, and with no real internal agreement within the military on how to proceed. Things actually boiled over during strategic planning sessions with the British. At the Casablanca Conference in January 1943 British Field Marshall Sir Alan Brooke accused American Admiral Ernest King (George Marshall's counterpart in the Navy) of favoring the Japanese War over the European theater (in direct violation of the Europe First agreements between the US and Great Britain).

That sets the basic stage. There was a clearly stated agreement between the Americans and British on a Europe First strategy. The agreement angered a lot of US military officers, particularly Naval Officers who wanted revenge for Pearl Harbor, and viewed the Japanese as a greater threat against US territory and civilians. There was also a rivalry between the Navy and Army on how to handle the Pacific, compounded by the presence of the oversized personality of MacArthur in the Pacific theater and his Philippines obsession. In this soup a command structure had to be determined, and resources allocated.

Chester Nimitz was the Navy choice for command of the Pacific Theater, and he was a brilliant choice. If he had been born 100 years earlier we may well have named a state after him. He was named CINCPOA, or Commander in chief Pacific Ocean Area and tasked with fighting a holding action in the Pacific with help from the army until Europe had been pacified. This decision worked well for no one except the British. Nimitz was a very junior Admiral with very limited resources tasked with fighting a holding action. For the Brits his appointment signified real commitment to the Europe first strategy. Everyone else had a problem here.

Nimitz was the first problem. He was a hyper competitive man, and a fighter. He wanted to stick it to the Japanese yesterday, and while he was careful with his resources he was committed to fighting an aggressive offensive war from day 1. I would say Admiral King, the Commander in Chief US Fleet and Chief of Naval Operations was in perfect agreement with Nimitz but he wasn't. King actually thought Nimitz and his officers were too cautious, and spent the early parts of the war constantly hectoring Nimitz to go at the Japanese harder. There was one other 800 pound gorilla in the Pacific room yet to be satisfied: Douglas MacArthur. MacArthur, and the Army Command structure in support of him wanted him to have equal stature with Nimitz, so the decision was made (a fateful decision, particularly for the Marines at Peleliu) to break the Pacific command up and make MacArthur commander of SWPOA.

MacArthur was given Army assets and ordered to defend Australia and prepare for the (eventual) offensive. A defensive command. Nimitz was given primarily Naval assets (and some army) and told to defend the areas under his command, but also to prepare for amphibious assaults. An ambiguous command that allowed for immediate offensive action. In allocating resources it's important to know that the Marines are not a separate branch of the service, they are part of the Navy, at least nominally under Navy Command, and aspiring Marine Officers attend the Naval Academy. In order to back door the Europe First strategy and start doing what they really wanted, which was to hit Japan where it hurts, Ernest King allocated all the Marine combat units, his personal army (as commander of the Navy) to the Pacific to be used for amphibious assaults. There were numerous amphibious operations in Europe, those were turned over to the army. King kept his Marine assets in Pacific where they would be under control of one of his admirals, and where he could use them to kill Japanese soldiers. There weren't many Marine units anyways, and the army brass liked the idea of picking up all those amphibious assets. Giving Nimitz carriers (to defend Hawaii) and Marines (to defend islands) was within the letter of the Europe First agreements, and the second the command structure was settled and resources allocated Admiral Ernest King went on one of the great letter writing campaigns of history. He was to put it plainly a complete asshole, and the worst boss ever to Admiral Chester Nimitz (while at the same time defending Nimitz to Washington and to the British allies with absolute conviction). King used threats, both veiled and overt, intimidation, insults, wheedling, lecturing, ordering, negotiating, dealmaking, you name it all in the service of impelling Nimitz forward into increasingly aggressive action against the Japanese. He also approved the Island hopping campaign, justified it as both a way to shore up the defensive lines against the Japanese and later to lay the ground for invasion of Japan, but everyone on both sides of the Atlantic soon realized that he was violating the spirit of the Europe First agreement with Roosevelt's tacit approval. After a few successes none of this mattered. With American troops in North Africa the British were mollified, and actions at Midway and Guadalcanal played so well in the US that supporting a 2 front strategy gained it's own inertia and lasted the duration of the war.

To answer it shortly, though (for anyone to make it past that long explanation) the Marines were diverted to the Pacific so Ernest King could backdoor Europe first and give his Navy a chance to take the fight to Japan right away, which he did while Roosevelt winked and looked the other way. With the few Marine assets locked down in the Pacific there were none to spare for Europe, which was fine because the Army had ample resources to send there and was willing to add amphibious assault to their repertoire.

Master of Seapower: A Biography of Fleet Admiral Ernest J. King by Thomas Buell gives a lot of background on these events and Admiral King's place in them.

By the end of the war, the Army was fielding more than 90 divisions. The Marine Corps had six. As you say, there were a number of Army units serving in the Pacific- because there weren't enough Marines to cover the entire theater of operations.

It's true that in other wars, Marines have served as ordinary inland infantry, but that's not their primary mission. The Corps is doctrinally intended to execute amphibious landings and engage the enemy in the littoral zone with support from naval air and artillery. In that sense, the Pacific was the perfect war for the Marines- using them in Europe (outside of a few operations) would have been a waste.