At their peak, the slaves numbered approximately 100,000 men, more than Hannibal's Carthaginians. Having enjoyed over a century of freedom from invasion, the Romans could not conceive of such a large force on their soil, never mind one built of slaves.
Crassus was one of Rome's most capable commanders, who had trained under a famous Roman general, Sulla. He later formed a "Triumvirate" with Pompey and the greatest of them all, Julius Caesar. Crassus was a ruthless man who performed the act of "decimation," killing every tenth man, of a unit that displayed cowardice. He re trained the Roman army into its later form (the one that Caesar used successfully) with throwing spears and short swords. He declined to fight the hard fighting rebels, when possible, preferring to outmaneuver them. Finally, he was reinforced by Pompey, another capable general, who brought his legions from Gaul.