Final answer:
The main factor contributing to the failure to find alternative labor sources during the transatlantic slave trade was the economic incentives for the slave trade itself. Slave labor was integral to colonial economies and high demand, along with insufficient alternatives, sustained the trade even after its formal abolition.
Step-by-step explanation:
The failure to find alternative sources of labor during the transatlantic slave trade was primarily influenced by economic incentives for the slave trade. The demand for labor in the colonies, especially after the depletion of indigenous populations due to European diseases, coupled with the economic motivation to exploit the New World's resources, necessitated a large, stable workforce that the African slave trade provided. Alternative labor sources, such as European indentured servants, proved insufficient and unsustainable in comparison to the magnitude of labor the slave trade could supply. Furthermore, African rulers and merchants were involved in slave trading, which facilitated the supply chain. The abolition of the foreign slave trade in 1807 did not immediately diminish the cruel institution but transformed it into a domestic practice within nations like the United States, reinforcing the dependent economy on slave labor for key industries such as cotton production.