I believe the answer is that there were more voters and laws to suppress those new voters. Post-civil war black people in the south were no longer enslaved as per the thirteenth amendment passed in 1865 (the exception to this rule was punishment for a crime). The fourteenth amendment passed in 1868 made people born in the United States citizens and subject to equal protections under the law, so formerly enslaved people were also citizens. The real kicker was the 15th amendment passed in 1870 that gave black people the right to vote. These amendments were called Reconstruction amendments and were part of rehabilitating Southern states upon re-entry into the Union. Now, states that seceded specifically to protect the institution of slavery wouldn’t like this new “black people are people thing” and there was a lot of bitterness on the losing side, particularly racist white bitterness. Post Civil War white southerners actively terrorized black people who lived in their states because the Reconstruction era was a threat to white supremacy. They began enforcing Jim Crow laws which mandated racial segregation and was upheld in the Supreme Court Case Plessy v. Ferguson. The Supreme Court rules that segregation was legal so long as the facilities offered to black people were equal in standard to what was offered to white people, but the equal part was notably not enforced. The Voting Rights Act of 1965 exists because White democrats were using the Klu Klux Klan and paramilitary force to prevent black people from voting. Many southern states passed laws that made voting registration and voting more difficult. The tests administered were often discriminatory. There were poll taxes and literary qualifications as well as a “grandfather clause” where you could only vote if you were a veteran or related to a veteran. This eliminated formerly enslaved people who would not have been able to enlist in the military. Many formerly enslaved people were also illiterate and did not have equal access to education because of aforementioned Jim Crow laws. The result? A Democratic stronghold in the south for 50 years. All white juries and further disenfranchisement of black people until the civil rights era. The Southern strategy that modern day Republicans use to this day as well as gerrymandering black neighborhoods is based on this template. Sorry for any errors I’m on mobile.

This question from a few days ago should be of interest.