Slavery began to develop in the Americas during the age of exploration because?

They needed slaves for labor in the New World.

The first great wave of expeditions was launched by Portugal under Prince Henry the Navigator. Sailing out into the open Atlantic the Madeira Islands were discovered in 1419 and in 1427 the Azores were discovered and both became Portuguese colonies. The main project of Henry the Navigator was exploration of the West Coast of Africa. For centuries the only trade routes linking West Africa with the Mediterranean world were over the Sahara Desert. These routes were controlled by the Muslim states of North Africa, long rivals to Portugal. It was the Portuguese hope that the Islamic nations could be bypassed by trading directly with West Africa by sea. It was also hoped that south of the Sahara the states would be Christian and potential allies against the Muslims in the Maghreb. The Portuguese navigators made slow but steady progress, each year managing to push a few miles further south and in 1434 the obstacle of Cape Bojador was overcome. Within two decades the barrier of the Sahara had been overcome and trade in gold and slaves began in with what is today Senegal. Progress continued as trading forts were built at Elmina and Sao Tome and Principe became the first sugar producing colony. In 1482 an expedition under Diogo Cão made contact with the Kingdom of Kongo. The crucial breakthrough was in 1487 when Bartolomeu Dias rounded the Cape of Good Hope and proved that access to the Indian Ocean was possible. In 1498 Vasco da Gama made good on this promise by reaching India.