What is the main reason for the rapid expansion of slavery in the South in the early nineteenth century?

The slaves provided labour in plantations.

Explanation:

Due to the fact that large expanses of land was available for farming and cultivation in the south, slaves were used as the fuel to grow crops like tobacco, cotton and sugar cane. Due to the fact that these plantations were making wealth, more labour was needed. So these plantations owners invested in more enslaved labourers since these labourers made them wealthy. These enslaved people provided more cheap labour.

answer; disconfirm the catharsis theory;