The main reason the Italian Peninsula was considered easy to defend primarily relates to its geographical location due to the presence of the Alps to the north. The Alps acted as a natural barrier against invasions from northern tribes, making it difficult for large armies to enter the peninsula. Additionally, the Apennine Mountains that run down the spine of Italy provide further geographical protection and make movement within the region more challenging for potential invaders.
While the other reasons mentioned have their merits, such as the military strength of the Romans and periods of stability like the Pax Romana, it was the geographical features that provided a significant advantage in defense.