Why were the beaches in France the most heavily defended beaches in Europe chosen as D-day sites as opposed to Holland?

The reason they were so well defended was that the Germans had every reason to believe that is where the invasion would come. Had they thought we were going into Holland they would have defended Holland. France in physically closer to England, That meant the invasion force could reach the invasion point more quickly and with less chance of being observed. It also permitted better air cover. Planes flying from England to Holland would have used up more fuel getting there and thus could have spent less time over the beaches. Furthermore, Holland is not called the Low Countries for nothing. Much of Holland is below sea level and protected from the sea by dikes. Those dikes would undoubtedly have been broken during the fighting and the resultant flooding would have vastly complicated operations.