During the second half of the 18th century, France opened up larger sugar plantations on the islands of Reunion, Mauritius and in the Indian Ocean. African slaves were thus recruited from East Africa to go and work in those plantations.
Africans were considered physically fit to work in harsh climatic conditions compared to the native red Indians and Europeans. This greatly increased the demand for the indigenous people(slaves).
The increased demand for sugar and cotton in Europe led to their increase in price and therefore more labour (slaves) was needed in the British colonies of West Indies and America.
Strong desire for European goods by African chiefs like Mirambo and Nyungu ya Mawe forced them to acquire slaves in exchange for manufactured goods such as brass, metal ware, cotton cloth, beads, spirits such as whisky, guns and gun powder.
The existence and recognition of slavery in East Africa societies. Domestic and child slavery already existed therefore Africans were willing to exchange slaves for European goods.
The huge profits enjoyed by middlemen like Arab Swahilli traders encouraged the traders to get deeply involved in the trade.
The suitable winds and currents (monsoon winds) which eased transportation for slave traders greatly contributed to the rise of slave trade.
The Legalization of slave trade in 1802 by Napoleon 1 of France increased the demand for slaves in all French Colonies.
The increased number of criminals, war captives, destitutes forced African chiefs to sell them off as slaves.
The Oman Arabs contributed to the rise in the demand for slaves. This is because they acted as middlemen between the African Swahili people,the Portuguese and French traders. They therefore worked very hard to get slaves in order to obtain revenue from them.
The invention of Spanish mines in West indices increased slave demands to work in the mines.
The exodus of slaves from East Africa to Northeast Africa, Arabia and Persia contributed to the increase in the demand for slaves. It led to an enormous number of slaves obtained from East Africa being transported to other countries.
The movement of Seyyid Said’s capital to Zanzibar led to an increase in slave trade. This is because when Seyyid said settled in Zanzibar in 1840, he embarked on strong plans to open up slave trade routes to the interior of East Africa. This boosted slave trade, whereby the number of slaves being sold at the slave market in Zanzibar annually by that time, reached between 40000 and 45000 thousand slaves.
The outbreak of diseases like Nagana led to an increase in slave trade. This is because the beasts of burden (i.e. camels, donkeys, etc) could not be taken on many of the caravan routes. It therefore necessitated people themselves to be involved in the transportation of the trade goods and ivory. Such people included porters who were regarded as slaves, or free Africans who could sell their services in return for cloth and other trade goods.
Development of long distance trade that needed slaves to transport goods from the interior of East Africa.
Plantation farming increased in some areas, especially the clove plantations were slaves worked.