IIRC the slave ships which visited the African coasts (they got their slaves by buying them from the victors of warring African tribes) went several places, not just to North America. They were much more likely to survive if headed for the American colonies/states, which accounted for something under a quarter of the slave business. None of this excuses the chattel slave business, but circumstances were propitious for those who came to America. When chattel slavery was abolished, few wanted to set sail back to Africa.
The Spanish and Portuguese brought many black slaves to the New World because they had a hard time enslaving the native Indians. It was very hard to force them underground to work in the mines and on the plantations. In some cases maybe the Indian population was too small.