A lot of the infrastructure of the South, like roads and freight railroads, were built by slave labor. Banks in the south grew from insuring slave owners' slaves and financing them. Insurance companies did the same. So many companies started in the south were started with capital the family had from slave labor. Some Universities and other institutions also were started with slave labor capital.
And then when the war ended and enslaved people were free, they started building their wealth and running for political offices successfully. It was those aforementioned companies, business leaders and institutions that lobbied and funded the following century's jim crow laws and terror to stop them from getting more wealth and power.