Why didn't the Allies place all their energy into a major invasion of southern France in place of the Normandy operation? German defenses apparently were much lighter there, as evidenced by the relatively easy time the Allies had when they came ashore a month or so after D-Day. I have an idea, but am curious as to what others think.
Any chance of surprise would totally be lost as the German's had spies in Spain and Gibraltar who would have noticed the thousands of ships sailing through the straits. Deception was a major component of the Normandy landings, and there would have been no chance for deception sending the ships from Britain to S. France.
Also the Allies would not have been able to put as many troops on the ground. The main bases in Britain held the million troops that were on the beach in Normandy by the end of July. Take Sicily as a counter example, we were only able to get around 150,000 troops there in the same time. This was approximately the same amount of troops that later landed in S. France. This was the operational limit in that theature. There was just not the infrastructure required to increase this limit. The supplies, the bases, and most importantly the ships weren't in the Mediterranean and any movement of that amount of material to the staging zones would have most definitely been noticed by German spies.
Also consider air power, it was much easier for the Allies to cover landings right next to our bases in the UK. It would have been much more difficult if the landings were in S. France.
also as jamesqf pointed out, the exact reasons the defenses were light were because they were being pulled up to N. France to deal with Overlord. Specifically the 2nd SS Das Reich and the 1st SS Leibstandarte Adolf Hitler were pulled from S. France to Normandy.
sources: troop numbers from Wikipedia (so take from it what you will)
sources for troop movements: http://www.iwm.org.uk/history/the-german-response-to-d-day