On the one hand, the far south was ripe for picking. It was a natural extension of our Sicily campaign and we had the troops there. Knocking one of the Axis powers out and taking its capital would boost morale on our side and diminish on the other. We will tie down units that otherwise would be available for duty in Russia or France.
On the other hand, it makes little tactical sense to go up the boot the hard way, just to get to the Po Valley. We would still be separated from Austria and Germany by the Alps. The tactically sound way to try it would be by a serious of end arounds, like Patton did in Sicily, but after the Anzio fiasco all sealift will be sent to Britain. It was a long and costly campaign that ultimately did little to end the war.
Air power. It dictated EVERYTHING the Allies did. We would not move one inch outside our protective air umbrella. At this point, the umbrella only extends to Salerno. It does NOT cover northern France, as the Luftwaffe is still alive, and would hotly contest the skies over an invasion fleet. All those fighters defending the Reich from B-17s would also defend the air over the beaches.
We aren’t going to invade France this year. But we can’t just sit around and do nothing, either. So Salerno it is...
Also one of the best German commanders, "Smiling Albert" Kesselring.