The best answer is mechanized factories and a steady supply of immigrants seeking work
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The Union held twenty-three states, including four border slave states, while the Confederacy included eleven states. The population count was about 22 million in the Union to 9 million in the Confederacy, and about 4 million of the latter were enslaved African Americans. An even greater advantage for the North was its industrial development. The southern states that formed the Confederacy produced just 7 percent of the nation’s manufactured goods on the eve of the war. The Union states produced 97 percent of the firearms and 96 percent of the railroad equipment.
