Why US Marines island-hopped while the Army prepared for Normandy

Did General Marshall keep the Marines out of Europe so they would not take the Normandy storyline?

No; Marines were committed to specialized Pacific assaults while Army formations prepared for the larger European invasion mission.

The division of amphibious labor followed theater requirements rather than a plan to deny the Marines credit for Normandy. Marine formations had become specialists in the Pacific island-hopping campaign, alongside a US Army division trained for similar work. The Army's larger European force was meanwhile preparing for Normandy and related continental operations. Operation Torch also shows that specialized commandos could lead the initial amphibious phase before conventional troops came ashore in later waves.