Assuming you're referring to the late 19th century, the political and economic mindset shifted towards capitalism and "free market" ideology, mostly because of the vast wealth that was created by the Industrial Revolution.
While the South utilized slavery to sustain its culture and grow cotton on plantations, the North prospered during the Industrial Revolution. Northern cities, the center of industry in the United States, became major metropolises due to an influx of immigrants. With this willing and cheap workforce, the North did not require a slave system. Although some northerners found the institution of slavery morally reprehensible, most did not believe in complete racial equality either. Slavery became even more divisive when it threatened to expand westward because non-slave holding white settlers did not want to compete with slaveholders in the new territories.
Jobs created by growing urban industries
The answer is rational basis
