The way I understand it is most of the slatve trade went to Brazil and parts of the Caribbean where African slaves were treated so badly that they didn't reproduce and had to be resupplied.

I've read that it was about 500,000 Africans brought to North America and that was mostly in the beginning. Relatively benign circumstances allowed natural reproduction in North America amongst the slaves.

I believe they did try other races as slaves

• North American Indians, Slavs, Irish, Caribs, but they couldn't hold up for the work. Having spent a few summers in Virginia I can well believe it. Working tobacco and cotton fields in that climate for a northern-born person combined with inadequate nutrition...Just didn't work out financially. I haven't seen any evidence that it was a direct attempt and diluting white Christian culture. In fact African Americans largely embraced Christianity after the second generation.