One reason for the assignment of the beaches was the location of the various armys in Britian. The US ground units were posted mostly close to the western ports like Liverpool. Many were in Ireland. The Brits tended to be towards the eastern side of the isle. With those locations reversing the beaches to be attacked would have required making the movement to the embarkation ports and subsequent sea transit even more complex. In war anything one can do to simplify is a good thing.