A lot of this has to do with the Cuban Revolution itself. At first when the July the 25th Movement came to power, it was not obviously communist. The effort to overthrow Batista had been the collaborative work of many groups including, trade unions, democracy activists, and Castro's guerrillas. The Revolution and Castro had broad support because he was able to turn the Revolution into a Cuban nationalist cause. Even when Castro started acting in direct conflict with American interests on the island, the popularity of the Revolution deterred the American military from recommending any overt military action because they feared a high casualty rate.

Korea and Vietnam were also different from Cuba because the US was defending pro-US countries that had been set up in the wake of WW2. The American military was protecting an ally from a communist aggressor or a communist insurgency sponsored by its neighbor. Cuba was not in this situation so any form of intervention would have seemed less legitimate.

Chomsky, Aviva. A History of the Cuban Revolution. Viewpoints/Puntos de Vista: Themes and Interpretations in Latin American History. Malden, MA, Oxford and Chichester, UK: John Wiley and Sons Ltd, 2011.