Answer:
Cause the Natives had died off. They needed slaves to do the labor of farming, so the transatlantic slave trade/the middle passage began.
Explanation: Afterwards comes about 300+ years of the most repulsive and inhumane trade possible. People are taken from their homes and treated as property. Most would die before they'd make the journey. 3-4months of grueling terror and anguish. It's something no one should bear or learn about, yet here we are, looking back into the dark past of the middle passage. It's a mistake that society might make again, and it's sad how true it is.