After the Civil War, the federal government was very wary of the southern states that had so recently rebelled. In came an era of Reconstruction, in which the federal government forced the South to give equal opporunities to blacks. Of course this didn't last long, and in came Jim Crow laws almost immediately after the Civil War. The South's economic development staggered.
Great Britain benefited from the policy still got their raw materials from the colonies and the colonies still bought the English finished goods. Great Britain's government longer must use soldiers or money to enforce laws and they can focus on the competition with France for total control of Europe.
