What was the economic reason for abolishing slavery?

One of the most important inventions in American history, was the cotton gin which made taking seeds out of cotton 50 times more effective than by hand. Thus, slaves were "needed" by the south to pick the cotton to fit the fast pace of the cotton gin. In fact, right before the cotton gin's invention, slavery was on the decline. Cotton in the south of the united states became a one crop economy, as 50% of all the exports of the United states, which was unheard of.