It was due to them all being in the Pacific. The Marine Corps only comprised of six divisions during the Second World War and by 1944 all six divisions were engaged in the Pacific Theatre. Even if it had been possible to withdraw some of these divisions and transport them to England, a difficult task even at the best of times let alone an active theatre with shipping at a premium, the Marine Corps’ expertise in amphibious assault was better utilised in the Pacific where amphibious assaults were commonplace as opposed to the invasion of Normandy which saw the US undertake only two landings. Additionally, US Army units were not without experience conducting amphibious assaults. The US had undertaken such landings in North Africa in 1942, as part of Operation Torch, Sicily in 1943 with Operation Husky and Italy in the same year with Operation Avalanche. Some of the units involved in these landings such as the US 1st Division also took part in the D-Day landings, bringing with them their experience of amphibious landings.

Basically, the Marine Corps wasn’t needed on D-Day. The US had enough units with prior experience with amphibious landings or training in the doctrine that the Marines were of more use in the Pacific where they would undertake multiple amphibious landings than they would in Normandy where they would undertake one landing then be committed to a standard land campaign or be transported back to the Pacific.