Basically: money 🤑💰. The desire for more of that led to all sorts of bullshit made-up "science" supporting the doing of what the people wanted to do anyway.

Here's a fantastic introduction video from what I consider a great series: https://youtu.be/Ajn9g5Gsv98. In particular it contrasts slavery in North vs. South America, where the latter was so much more violent and bloody and dismembered so many more people that they had to constantly import more bc so exceedingly many died that they could not keep a self sustaining population of them, which somehow still made the northern version all the more evil even while being more "gentle" bc they could therefore breed slaves like cattle.

The dehumanization might have been part of the point, or it may have only been helpful to make the money, but either way, the families of slave owners who did not go out into the fields and therefore were not as closely associated with it - though crucially, still directly benefitted from the work product - went along with it all as collaborators due to the fact that their financial status depended upon them doing exactly that.