Answer:
It decreased because they realized that it was safer to reunite small amounts o slaves to develop labour instead of large amounts o slaves because they could uprise and represent a very big opposition.
Step-by-step explanation:
The Zanj Rebellion in southern Iraq in the 19th century brought up a big problem for agriculture and the social order established in the Era. Because in the first place, it was so big many people heard about it and started to understand big amounts of slaves in the same labor and place would create another uprising. In the second place because there was a big competence from slavery in the new world, so increasing slavery to counter the numbers o the lost slaves due to death would not result in a very competitive effort. And in third place, because many slaves learn from it and they were really prone to create another uprising.