The main reason for the lack of protestant reformation in the south was the Papacy. Papal power was strongest around the Vatican and Rome, and the Papacy's Italian allies, and lessened in power the further north you went. This meant that those southern states, especially those allied with the Papal States, were far less likely to want to directly oppose the Pope by converting to Protestantism.

You could also look at why did it spread so quickly in the North. Again, this is partly due to traditional opposition between the northern nobles of the Holy Roman Empire and the Papacy. There had been conflicts for centuries before the reformation between the Pope and the Holy Roman Emperor and other germanic nobles. This opposition could then naturally make these areas more likely to reform in opposition to the Pope.