Slavery just was not used in factories, historically slaves were used in RGO and mostly in places were there few population and manpower was required. In China, India, Rusia, slaves weren´t really needed.

But back to factories, slaves just wasn´t used in factories, I have theoretical fundaments, but for short sake, just in history and real life, slaves were used in RGO but not facttories.

Also slavery should be more inefficient than employ lanbourers, as historically, when population was available, slavery became serfdom and was abolished de facto.