Why was the Atlantic Slave trade so entrenched in the economic systems of the colonies, Europe, and Africa?

During the so-called triangular trade that ensued from European colonization of Africa, slaves were purchased in West Africa
subdue inland ethnic groups in order to establish complete colonial rule and to exploit mineral resources. In the 15 and 16th centuries, slaves became the major commodity for European trade. The Portuguese began the slave trade around 1510. The Dutch took over all Portuguese possessions along the African coast in 1642.