The English Channel is not very wide, only about 90 miles at the closest points between England and France. That distance doesn't seem like a whole lot, but when it comes to military invasions it's a formidable task. The Allied invasion consisted of over 7000 vessels departing from different staging sites all around England. That meant the vessels would be traveling different distances and, because of the variety of craft used, at different speeds. All these vessels had to be coordinated on very strict time table to arrive at their beaches all at the same time. It was a monumental task and it's a great credit to General Eisenhower, who was the supreme commander, that the plan was well laid and well executed.
Add to that the brilliant deception by the intelligence agencies that had lead Hitler and most of his generals to believe the landing would be at the Port de Calais. The ruse was made particularly credible because that was the shortest route the allies could have taken and the one that Hitler's staff would have picked for such an invasion. General Rommel had that same view, but with a note of caution. He recognized that Normandy and the other beaches were possible targets, too. Which is why he tried to set up a defensive perimeter along the entire coast line. His view was that wherever the allies struck they had to be beaten on the beaches. The Germans couldn't count on making the right guess. They had to be prepared for anything. The allied air superiority at that point meant that if the allied troops got past the beach they would have the upper hand.
Logistically it was the most challenging invasion in human history, and many historians count it as the most challenging invasion overall. Hopefully humankind will never have witness such an event ever again.
Side note: The English had long considered the channel as their best first defense against invasion by the French. When the tunnel under the channel linked the two countries many older English citizens were a bit ticked off. Why make it any easier for the French to come?
