You raise a good point. Slavery of the intense proportions seen in the Free Cities was only seen historically in single-export plantation economies. And yet, the Free Cities’ economies are overhwelmingly focused on commerce, finance, and the production of high value-added manufactured goods.
So vast latifundia in the Disputed Lands would provide at least a partial explanation for why you see such unusually high levels of slavery in the Free Cities. Not a complete explanation, but better than what we have now.