Because he had been doing so well, a lot of his army wanted to stay in Italy and loot and pillage some more, reaping the sweet rewards of victory. In Plutarch's Life of Crassus says that Spartacus "began to lead his army toward the Alps, thinking it necessary for them to cross the mountains and go to their respective homes, some to Thrace, and some to Gaul. But his men were now strong in numbers and full of confidence, and would not listen to him, but went ravaging over Italy", giving the impression that his men did not wish to flee over the Alps, but instead continue pillaging in Italy, such as a can be seen in the case of Crixus who split from Spartacus's main group with 30,000 men. As for why Spartacus himself didn't just flee, but turned back to Italy with his men, there is no definitive answer. One possibility would be that he became overconfident as well, but there's no proof. In Appian's The Civil Wars, he describes how upon hearing of Crixus's defeat and death at the hand Consul Lucius Gellius Publicola, Spartacus forced 300 captured Romans to fight to the death in gladiatoral combat, which might suggest an element of revenge in Spartacus's decision to turn back.

Source: Plutarch Life of Crassus, Appian The Civil Wars, Barry Strauss The Spartacus War