Nkrumah Was Right
Superstar
I wonder why they're more prominent in Uruguay than Argentina. Proximity to Brazil?
Below is a map of South America. The closer you are to the equator the warmer the temperature is and vice versa.
Europeans enslaved millions of Africans, in part, in chattel slavery to work crops which require warm temperatures: sugar, indigo, cotton etc.
In summary, far less enslaved Africans went to Argentina because Argentina is colder than Uruguay. Also, Uruguay (as you noted) is proximate to Brazil.