This is a good question but your post makes a number of assumptions which it really shouldn't make.

First, slave labor was predominantly (but not exclusively) associated with agriculture. Slaves were mostly used to raise crops like cotton, sure, but also rice, indigo, tobacco, and others. Most of the US's industry was located in the North. In the first few decades after the Civil War industrial output in the US skyrocketed, but most of that was in the North as well. The South would undergo only a modest shift toward industrial pursuits.

Next, slaves weren't living the good life by any means but their masters paid great attention to the perceived optimal ways to maintain their slave populations. Food that didn't nourish them, clothes that didn't keep them warm, and shelter that didn't protect them from the elements all meant that slaves would produce substandard work. Slaveowners certainly weren't overly concerned with the well-being of their slaves (not nearly as much as they claimed to be, at least), but they didn't adopt a "let them eat cake" attitude either. Slaves were highly valuable property, after all.

Finally, it's wrong to assume that conditions suddenly and drastically improved once the war ended, or that the monetary gains by freedmen (ex-slaves) were much higher than what they received under slavery.

For that, and a lot of further detail that will help answer your question, I'm going to link to a similar post made recently where I gave a decently detailed answer.