As someone else said, the Marines were already engaged in the Pacific War and there was no good reason to repurpose them to land in Normandy, given the logistics involved (transporting them half way across the globe, for starters). I guarantee no US General said "Well, a beach landing is what the Marines should be used for, but Normandy is going to have high casualties, so let's just use the Army as cannon fodder and spare the 'elite' Marines so we can use them in the Pacific." The fact is that it just made no logistical sense to use the Marines there when the Army could do the job just as well and the Marines were charged with fighting in the Pacific. I guarantee you that if an equal number of Marines had landed on Normandy rather than the Army, the casualty figures would've been more or less the same.
Also, no offense to anyone, but I've frankly never understood the perception that Marines are elite. For sure, you have to be in top physical condition and be very brave to land on a beach with fire coming down all around you. But as someone who has met both Army soldiers and Marines, I never could tell much difference in my interaction in terms of the quality of the person. I've known two Marines directly -- an uncle and my brother's friend, and both were complete idiots and that lowered my opinion of the Marines. So, please educate me on the why the Marines are so elite.