Slave labor was used in the Southern colonies because
The main reason it was allowed was because slave labor allowed farmers to plant and harvest crops without having to pay workers for their labor.
Hope this helps:)
pretty much exactly as the other person said; slaves worked on plantations to help plant and grow crops. the slave owners didn't pay them so the people who owned the plantations made a huge profit. the land in the southern colonies were also very rich and large in size.