What economic factors best explain the pattern of the expansion of slavery?

owners of slaves do not have to invest into their slaves and pay minimum for them. So it was - from purely economic point of view - advantage to have slaves. White workers would require much more. Slaves were viewed as pure objects and they themselves accepted that position. Moreover, slavery was also supported by the church. Slavery expanded because of this, especially in the south and in tropics.

Explanation:

2686-2181 bce-- "egyptian civilization developed along the nile river in large part because the river’s annual flooding ensured reliable, rich soil for growing crops."

1069-664 bce-- "ancient egyptians developed wide-reaching trade networks along the nile, in the red sea, and in the near east."