D-Day was a risky choice, as most defences were on that part of the coast - even if Normandy was slightly less defended. Attacking southern France only would have been another problem, as the area is really mountainous, while once you've pierced the "bocage normand", the soil is flat till Berlin, minus 2 or 3 rivers.....

When the S-France landing did happen, German defences were already stretched, so the french army over there did advance pretty quickly - with the precious help of US air force. But with all the defences there, the area would have been overcrowded, & really tough to advance in. D-Day was the risk to take. Congrats Mr Eisenhower.