They are afraid that if Cubans ever get a taste of capitalism they will throw off the commie yoke. Go to you tube and look at the videos of SVN today. The communist dropped a lot of restrictions on capitalism and the country has really changed for the better.
Democracy doesn't work in a society whose primary unit is the tribe or clan or family rather than the individual. South Korea seems to be the one real exception to that. In Viet Nam it is the family. Were there to be serious elections people, no matter what they believe as to how the system should be run or what the qualifications of the candidate, would vote for the candidate nearest in relation to the voter. That is true of much of the non Western world.
That is why it is futile and a terrible waste of resources to promote Democracy in the third world. Right now Viet Nam probably has their optimal government. The US (pace Obama) also has the best government in Viet Nam that the US can get. That Communist form includes a very large army with which to intimidate Peking. Peking could, of course defeat Viet Nam in a war but it would be far more expensive to China than China deems the project to be worth. 1979 was a demonstration of that even though China "won" that fight.