As industrialization picked up, why didn't factory owners use slaves to staff them?

As always, u/Tatem1961, you are asking the cool questions about Atlantic history!

So, the Southern (USA) conceptualization of slavery and its expansion and ultimate survival varied somewhat considerably between time and geographical region. The best explanation we have for this is that different enslavers and their political allies (that is, poor whites from these communities who served as the voting block for them) tended to understand slavery differently based on which time period and location they were in.

Deep South planters at the turn of the 19th Century and their ilk tended to focus more on the plantation and the use of extraction economies. What this means is that these people were so deeply caught up in the massive profit quantities which were output in the years leading up to the Civil War that leading Southern politicians really didn't consider industrialization to be necessary (this is where I wish I could link you my own research, which focuses on how white conceptualization of society and economics fundamentally drove the direction at which development in the South was spurred, but I know using myself as a source is not an example of good scholarship).

Of course, there were industrial centers in the South - New Orleans and Richmond come to mind - but they were nowhere near the scale that existed in, say, England at the same time. A Deep South planter - the ones with all the money in this place at this time - were really just not interested in trying to teach the individuals they held in bondage how to do the often skilled and/or dangerous labor which required working in factories. Besides, there were plenty of white folks who needed paying jobs, and it served these people better to have the white folks working in factories because, well, chattel slavery seemed to be incredibly profitable and moving resources around would result in growing pains and pitfalls. Ultimately, though, there were some plans and actions taken to get enslaved people to work in the factories which were left starved of white laborers due to conscription of white men into the Confederate army. The best-known example would probably be the Tredegar Iron Works, which produced virtually all of the domestically-produced munitions for the CSA army. Tredegar had been using enslaved labor to some extent since the mid 1840s.

However, there was a conceptualization of slavery which attempted to adapt bondage as the world entered an industrial era. Henry Clay is most often associated with this idea. He thought slavery could exist for another 50+ years beyond the Antebellum period if enslavement could "adapt" with the changing world. Instead of enslaved people being held in bondage on plantations, they could work in the mines, work in the timber industry, and work in factories. Clay thought that the South could adapt and keep slavery alive in this way. Frankly, he wasn't exactly wrong: industrial slavery did exist, just not on a scale anywhere near agrarian or city/ "domestic" slavery did.

Sources:
Markets Without a Market Revolution: Southern Planters and Capitalism by Douglas R. Egerton

Labor, Race, and Technology in the Confederate Iron Industry by Anne Kelly Knowles

American System by Henry Clay

Western Lands and the Political Economy of Henry Clay's American System, 1819-1832 by John R. Van Atta