IIRC, survival rates in the United States were much higher than in South America. We outlawed the import of slaves in 1807 or something - so the slave owners tended to take better care of their “property” here than other places.
The slave traders also lost a lot of slaves in Africa as they were marched to the port, in the port holding camps, on the long ship journey, etc. In S. America the slaves were sent to camps to be “broken” prior to sale (that didn’t happen in N. America for some reason).
I don’t think the U.S. ever outlawed the import of slaves. Now the British outlawed slave trading at some point in the early 19th century but I not aware of the other by the U.S..