There are a few reasons. One is simply that Batista's army was badly run and organised and had a huge morale problem. In very stark contrast to Castro, Che, et al., Batista was not well liked by his rank and file, who mostly came from the peasantry and urban poor -- i.e. the classes that suffered most under his dictatorship. As the war wore on, defections became more and more common. The guerrilla army grew as Batista's shrank.

A badly organised army with a morale problem was a godsend for the guerrillas, who -- in the early stages of the war, at least -- had to rely exclusively on the enemy for weapons, ammunitions, and other supplies. Surrender was common and a win-win situation for the rebels: fewer shots fired = more ammo conserved on both sides = growing stockpile of weapons and ammunition. It was a political win too: Castro couldn't claim to speak for the lower classes while killing them en masse.

Another extremely important reason for Castro etc's success is that, although the rebel army was massively outnumbered, it enjoyed widespread support among peasants, who aided the war effort by providing the rebels with food, shelter, storage space, and (crucially) information. For the army, this made the already extremely difficult task of tracking the guerrillas even harder. (For comparison's sake, the Bolivian peasantry feared Guevara's guerrillas in 1968 and look how that ended up.)

The fact that the US didn't get involved in the war in any significant way was a big plus for the rebels too. Batista wasn't popular in Washington and Castro was smart enough to keep his politics ambiguous until he consolidated power, by which point Uncle Sam could take a flying leap.

Finally, the mountainous, jungleish terrain of the Sierra Maestra, where a good chunk of the war was fought, was perfect for guerrilla warfare. It made large troop movements impossibly cumbersome, gave the guerrillas plenty of places to hide, made it almost impossible to track them on ground or by air, and was basically custom made for ambushes. Even with today's technology, it's not easy to find a motivated, well-trained guerrilla band in inhospitable territory. See e.g. Afghanistan.

Sources: Anderson's 'Che Guevara: A Revolutionary Life', Castaneda's 'Companero', Guevara's Reminiscence's and Guerrilla Warfare.

Do you mean the rebel movement in the 1950s?