The blockade on Cuba is maintained primarily on the basis of wanting to change Cuba's government and economic system to mimic those of the United States. However, other countries who have different types of governments and economic systems, whether similar to Cuba's or not, have not received the same kind of treatment Cuba has from the US.
Though the US Government says it wants democracy for Cuba, it ignores many governments in Latin America silence their political opponents with violence.
Fidel Castro is occasionally referred to as being the world's last communist. However, China (with a quarter of the world's people), North Korea, Vietnam and other socialist or communist nations still exist.
Although there has been plenty of American aggression towards these nations, the US does now trade heavily with China.
President Clinton's recent visit to China affirmed the trade relations between the two nations. The US alleges the same problems exist in China that exists in Cuba, yet they take a noticeably different attitude towards the countries.
So, why does the United States treat Cuba specially?
The primary reason is that the US is heavily influenced by Miami exiles who oppose Cuban trade. Many Presidents have given in to them in order to win Florida's electoral votes, and have received funding for their campaigns from these groups.
Another possibility is that Cuba does not provide quite as strong an economic opportunity as China. For access to China's huge population, America is willing to forget its differences with the Chinese Government.