Why were African slaves needed for slavery?

Slaves have been used throughout mankind's history. They historically come from people abducted by force, people given away or sold by their leaders, people who sell themselves or people conquered in war.
African slaves were used in America and Europe, primarily because their tribal leaders and kings sold them to the slave traders. Many other races were sold into slavery at the same time, but not in the same numbers.
Scholars estimate that about 12,000,000 Africans were sold by Africans to Europeans (most of them before 1776, when the USA wasn't yet born) and 17,000,000 were sold to Arabs.
Slavery still exists in many countries today.

You have asked "why". Because it was (and still is in some places) possible to buy people as servants and workers.