No one knows exactly why but the show implies it had to do with deferring ambitions...
Spartacus and Crixus both wanted freedom but Crixus believed the only way to truly achieve that was to sack Rome...Spartacus knew that was impossible, he felt he had a better chance of achieving freedom by leading his people away from the grasp of Rome hence why he tried to get the pirates to help him ship his people off to Sicily which wasn't under Roman control...Unfortunately he got betrayed and had no other escape route...According to history Crixus split before the pirate betrayal, on the show it was after...Either way they were doomed though, Rome was too big to outrun and they never stood a chance against the legions Crassus commanded...
we have to remember pompey sealed the deal by getting the slaves who were trying to escape and basically bottled in spartacus