This is the correct answer. Most answers here focus too much in technology, but forget about economics. And even if they had more technological advances, it is

This is the correct answer. Most answers here focus too much in technology, but forget about economics. And even if they had more technological advances, it is difficult for a technology to became competitive when you are competing with slave labor. And if slaves are supposed to operate your technology, this also creates several technological restrictions: slaves always will treat their working tools badly, so you cannot have machines with delicate parts.