The US slave owners generally wanted the slaves to breed because it was cheaper than importing them I suppose. The same way you wouldn’t kill off the cows, horses and field hands - just bad for business.
There is also the fact that North America was the most distant market for slaves, and therefore they were most expensive there.
For Caribbean and (especially) Brazilian slaveowners, it was cheaper and easier to just work slaves to death and buy new ones.
Anywho, the fact that slaves were able in America to survive and thrive sufficiently to reproduce in large numbers indicates by definition that conditions were much less harsh there than in the other market areas.