The D-Day landings were an amphibious operation basically for a matter of hours. After that, the campaign was largely a conventional land operation (the "amphibious" element after the initial landings was largely a logistical matter, well within the Army's capabilities). The USMC was also much smaller than the Army (approx. 7 divisions versus 80-some.) Rather than hamstring the Pacific war by transporting Marines halfway around the globe for one small phase of the invasion, it was simpler to train the Army in invasion techniques. To this end, Marine advisors were attached to Overlord for technical assistance. The Marines stayed in the Pacific where almost all the fighting was Marine-style.

Politics played some role as well. The USMC, and adjunct of the Navy, stayed in the Pacific where the Navy was the prime service. Europe was the Army's game.

u/Prufrock451 answered a question very similar to this some time ago.