Why did Fidel Castro win?
We all know the answer. Because the U.S. wanted him to win. But I don't mean that.
What personal attributes facilitated his rise to power?
His aptitude for mendacity and deception, of course. His histrionic vent. His Nietzchian sense of "destiny" which he shared with other 20th century totalitarians. His instinct for self-preservation. And his bloodlust. Especially his bloodlust.
Castro didn't prevail because he was courageous; no more cowardly a man ever lived. By the age of 20, he had killed three men. All shot in the back. Castro was a serial killer before he became a mass murderer.
Of course, personal courage was never a requisite for attaining power if you surround yourself with proselytes willing to do the dying for you, which he always did.
That he found such men is itself remarkable because loyalty is a foreign concept to him. Ruthless as he has always been with his enemies, Fidel Castro has been a worst friend than enemy if it can even be said that he was a "friend" to any man.
We could inventory all the human virtues, and find, in the end, that he possesses none.
Then why did Fidel win?
Because he is a man without principles and the very incarnation of opportunism. That is his lesson and his legacy to his countrymen.
To their credit they have not learned the lesson or accepted the legacy.
There is no more compelling proof of this than the general rejection of Raúl Castro's proposal to swap the 5 (anti-)Cuban spies in U.S. jails for Cuba's 220 or so internationally-recognized prisoners of conscience and their families (there are, of course, a thousand times that number imprisoned in Cuba for asserting their rights as humans which human rights organizations have not the resources to "adopt" and perhaps not even the discernment to recognize).
Apples are routinely traded for oranges but not in the moral universe. That is a place that Castro has never inhabited or even visited. The Cuban people, who are neither opportunistic nor unprincipled, know the difference between heroes and henchmen. A prisoner of conscience will not act against his convictions to secure his freedom. A common criminal, on the other hand, would consider dishonor the cheapest price that could be paid for freedom.
Castro, when he was briefly a prisoner, never tired of petitioning Batista for an amnesty through family and political connections, and made it clear himself that he would not refuse it were it offered to him. In fact, Castro did accept Batista's amnesty, which necessarily entailed the recognition by him that Batista, re-elected in 1955, was the constitutional president of Cuba and hence empowered to decree an amnesty freeing him and his cohorts from jail. Here was the most vocal critic of Batista's legitimacy suddenly recognizing that Batista was Cuba's legitimate president when exercising his authority on his (Castro's) behalf. This was much worse than a prisoner swap which would not entail the recognition on either side of the legitimacy of the other. Castro's opportunism allowed him to grovel before Batista (or at least to allow his wife to grovel) and his lack of principles made it quite easy for him to buy his freedom at the price of betraying everything and everyone whom he claimed to represent.
And still he won.