Modern capitalists prefer seasonal labor for agriculture. They don't have to feed/clothe/house people year round, and have no personal investment. Seasonal migrant agriculture labor cheap and easily exploited, with little legal protection. Slaves, like domestic a nimals (reprehensible as that simily is), must be treated well enough to keep working productively. There is no such need with migrant labor. If they are abused or killed it is easy to sweep under the rug. There'll be new migrants available next year.
NOTE: I'm not saying slavery is good, or even better than migrant labor. They are both highly unethical if you consider how corporations treat migrant labor today.