Original: The Third Servile War was the only one to directly threaten the Roman heartland of Italy and was doubly alarming to the Roman people due to the repeated successes of the rapidly growing band of rebel slaves against the Roman army between 73 and 71 BC.
Simplified: The Third Servile War was the only which was dangerous for Italia itself and was doubly alarming to the Roman people because the slaves won several battles against the Roman army between 73 and 71 BC.