To put it simply, Normandy was the best option available. It was close enough to the UK to make transporting troops and supplies or providing air cover relatively easy, but not so close that it was an obvious place to defend. It had good beaches, and easy access to port facilities in Cherbourg. It was less well defended than places like Calais, but it was more accessible than places like Brittany that were less well defended. For more information, see this comment where I discuss this in more detail.