I'm surprised that a nice moderate republican like herself had such a poor education.
First, the slave labor you talk about which includes indentured servitude primarily benefited the British Empire not the USA which didn't even exist until more then 150 years after slavery was brought to the COLONIES.
Second, at the birth of the USA the north had a stronger and more balanced economy then the South did. The North already was the leading manufacturer in North America and had a robust shipping industry (both building ships and transporting goods). Importantly the Norths agricultural sector was able to feed the people of the North.
In the years between the Revolutionary War and the Civil War the North steadily progressed towards becoming an industrial power. The "wealth" of the South was almost entirely due to the export of cotton and existed in the form of slaves, land and lavish plantation infrastructure. None of these had a big influence on the wealth and growing industrialization of the North.
In many ways the North & South were almost like two separate countries with one being held back by the burden of slavery and the other being free to innovate and adopt the industrial revolution.