Why was Africa the center of the slave trade?

Trade in human slaves began in the earliest times and went on until the nineteenth century. But it was in the sixteenth century, after the discovery of America when the need grew for labour on the new plantations, that the slave trade began to increase. Its center was Africa. Millions of slaves were transported from that continent. The British, French, Dutch, Portuguese and Spanish all engaged in the trade. There was also a trade by the Arabs had a long history of slavery, and their African traffic in human merchandise was only suppressed towards the end of the nineteenth century. After the trade had been made illegal by western nations, Arab dealers in Africa continued to buy and sell people. They took them from the interior of Africa in caravans to the shores of the Mediterranean, the Red Sea, the Persian Gulf and the Indian Ocean. Some were sold at the coast but most were shipped on to Turkey, Arabia, Iran and other eastern lands. The biggest slave market in those days was in Zanzibar. In 1845 the British reached an agreement with the Sultan of Zanzibar by which his territory ceased to be the center of the traffic, but it persisted on the African mainland and was abolished only gradually.