I'm aware of Dragoon, which took place a few months after the Normandy invasion, but why was the invasion of Southern France such a side show compared to the D-Day invasion? What was it about invading from the south that made it unfeasible as a route to the eventual invasion of Germany and winning the war?
Part of the decision was England. The British provided a perfect staging ground, on friendly territory, and they had the infrastructure and chain of command to easily assemble an invasion force. In addition, landing in Normandy and continuing east would push the German army away from England, which they had been threatening to invade from across the Channel for most of the war.