Dystopia in a Southern Spanish Dixie?

Is it possible that what would become Argentina and Uruguay could develop more similarly to the US south than Brazil in terms of a slave-based plantation economy? It seems that the climate would have been appropriate for some crops, and I know that labor was very expensive in the region, even compared to North America.

My only guess is that it didn't happen as the Spanish didn't develop the area for such a purpose, and that settlement was somewhat slow compared to other parts of the Spanish Empire, or even Brazil. Are there some other reasons in particular?
 
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