That's a question that I have asked myself many times and based on what I have found out through research and asking a few history professors at the University of Connecticut, this is what I found.
It turns out that the Spanish Colonies differed in their clasification of slaves from the Americans. Apparently, Spanish Colonies recognize slaves as human beings whose freedom was automatically granted once their master died or if their master wished upon it. Also, the off-springs (this was only in effect in Cuba, Puerto Rico, and Santo Domingo) of the slaves would automatically be free.
In the USA, the constitution never recognized slaves as humans, instead slaves were property in the full extent. Meaning, a slave had no rights what so ever, also off-springs of the slaves were not free. Slaves could have also bought their freedom or get their freedom at their masters will, but seldom did that happened.
That was the biggest difference which caused difference in how slaves were seen in the different areas controlled by the Americans and/or Spaniards.
In my opinion, the fact that for about 100 years after slavery was abolished in the United States, the US still practiced a form of Aparteid where blacks and whites were segregated in all areas. For many years after most (if not all countries) of the world where slavery had been practiced, after they gave the freedom to their slaves, the slaves were free to do what they wanted and people were not segregated, at least by law they weren't. Stories and news about the segregation in the US and the many lynchings, abuses, and human rights violations towards afro-Americans, in my opinion, probably helped in imbedding in the world's minds that the Americans were in more ways than one much more responsible for slavery and negative connotations towards blacks or something. I know that's not true, but that could have been a reason why the US is always singled out in this slavery issue.
I think that its important to note that Santo Domingo was the first place in the new world where the enslavement of Africans first began. When the Spaniards moved their empire Capital from Santo Domingo (after they ceded the western third of Hispaniola to France) to Havana, then Havana became the main slave trading port in the hemisphere. Most of the slaves in Santo Domingo were either sold to the French in St. Domingue (Haiti) or given their freedom, with few actually being enslaved.
I think its also important to note that eventhough the US was an important trading partner of Havana in the slave trade, St. Domingue (ie. Haiti) was their biggest customers due to the EXTREMELY harsh treatment the French imposed on the Africans. In fact, thats where Africans had the lowest life expectancy after being enslaved, in St. Domingue the average slave only lived for 6 months after arriving at the port of Cap-Francais (today Cap-Haitien) or Port-au-Prince.
I like to point that out because the USA WAS NOT the worst player in the slave trade, there were others (French) that were much more brutal to the slaves. In addition, the Dutch were the ones involved in the actual transportation of the slaves from the different ports along Western Africa to Havana.
It's also important to note that Brazil (under the Portuguese) had way more African slaves than the US had! But, I guess that the fact that the US segregated afterwards (something that didn't existed in other countries in the New World after slavery ended) may have contributed for the world to look at the US as the evil doer when it comes to slavery, eventhough there were worst players in that game. I'm not saying that what the US did was nice or good, it was still wrong, it's just that in my opinion (and I'm not an American by the way) in my opinion America is being blamed far more than its fair share in this slavery deal.
