What is one reason "most rebellions aboard slave ships were unsuccessful?"?

There were many reasons why most insurrections aboard ships were unsuccessful. However, it did not include the concept that slaves were often taken from different regions of Africa.

Escaping ships were difficult. The European sailors would anchor on coasts and fill the ship with African captives. The ships were floating prisons. The Ship's crew would stay alert. They were outnumbered so they would use chains and shackles to control the African captives.