The Italian peninsula had many great advantages.
Firstly, the peninsula has a natural water barrier to deter anyone from attacking from land. If someone would want to attack it would have to be from sea or they would have to force an attack through a choke point on land.
Secondly, the mountains on the peninsula were quite advantageous for more defense and when the Tiber river flooded there was somewhere to go.
Lastly, the peninsula allowed for more trade to open up because it was so easily open to water. The many bustling towns and city's on the peninsula made it a economic hot-spot.