Why it's the capital
Rome was the capital of the Roman Kingdom, then of the Roman Republic and the Roman Empire. In 1871 it became the capital of the Kingdom of Italy, which became the Italian Republic in 1946.
Capital of ItalyRome is in the central-western part of the Italian Peninsula, in the Lazio region, on the River Tiber about 25 km from the sea at Ostia.
Rome was the capital of the Roman Kingdom, then of the Roman Republic and the Roman Empire. In 1871 it became the capital of the Kingdom of Italy, which became the Italian Republic in 1946.
Picture Italy as a boot: Rome is about halfway down, on the western side, a short way inland from the Tyrrhenian Sea.
Vatican City, a separate country, lies inside Rome, the only country within a city. On the globe the two capitals are almost the same spot.
Sources: Wikipedia: Rome, Wikipedia: Tiber, Wikipedia: Colosseum