Why was slave labor used in the southern states of the United States, in contrast to the northern states?

In the southern states it was mainly unskilled monotonous labor that was required to pick cotton, which was the “gold of the Confederation.” There was a lot of cotton, so a lot of work was also needed. If every worker had to pay, then the sale of this crop would not bring such a colossal income (and we should not forget about the cultivation of other plants, livestock, the maintenance of plantation houses). The South was a rich and prosperous land precisely because it used free slave labor.