Why did the Germans deploy heavy troops in Calais during World War II?

Because the Allies engaged in a great strategic deception, first of all, they collected a lot of weather and maps in Calais, and prepared a lot of tanks and plane models on the other side of Calais to deceive the Germans and make them think that the Allies would land in Calais. So the German high command was fooled and arranged the elite German troops in Calais. Only three top generals thought that the Allied forces would land in Normandy.