Spartacus was very successful initially, and won several battles against the Romans. In fact at one point the road was clear and his army could have escaped to the north and out of Italy. For reasons that are now lost to history, instead of continuing north the army turned south and ultimately to its doom. Perhaps they got drunk on early success and plunder and got overconfident.
Or perhaps they simply didn't know the way. We're talking about the era before widely-available accurate maps. Even the average citizen of Rome probably wouldn't be able to find his way out of Italy without a considerable amount of help. Spartacus was leading a collection of
slaves, with a proportionately lower propensity for adequate navigation.
We also don't really know what the insurrectionists' objectives were in the Third Servile War. Perhaps they were trying to escape Rome and flee to Gaul or Pannonia or wherever. Or perhaps they were intent on bringing down the Republic. The period historians differ on that point.
And finally, there's not a lot of evidence that Spartacus actually
was a revolutionary in any meaningful sense. No source records him as attempting to end the institution of slavery. Theoretically, revolutionaries are fighting for some idealized future state, even if they don't know the mechanics of implementing it, but there's no particular reason to ascribe such a motive to Spartacus or the others in the absence of evidence.