Why Was The Slaves Africans?

Because it made economic sense -- to slavers.  The West of Africa provided a ready source of people.  There was no more convenient source to meet the market demand.
Slavery has been a fact of life in Africa for thousands of years and is still practised.  All that changed in the 16th century was the sheer scale of the trade.  Deals were done on the West African coast with local tribes or slave traders, who were powerful and very well rewarded.  They raided areas further inland, usually rival tribes or weaker tribes to provide slaves for the willing buyers.  They organised their capture, imprisonment and effectively, got them ready for sale.  It was organised commercially on a huge scale, with ships regularly carrying hundreds of slaves in hideous conditions.  The main markets were in the new western colonies in the Indies and America's southern states where they were used on the cotton, sugar and tobacco plantations owned by colonists from France, Spain, Portugal and Britain.

We mustn't forget though that for 400 years and more before that slave trade began, the Barbary pirates (Barbary refers to the coastline of modern Tunisia, Morocco and Mauritania) had their own reign of terror.  They regularly raided the southern Mediterranean coast of Europe and the western European coast, even raiding as far north as Southern Ireland and the south coast of England.  Their raiding parties kidnapped slaves for slavery in various parts of northern Africa.  It was known as White Slavery.