Because the African Americans who did show up to vote were told by white basically henchmen that their time to vote passed or that they were at the wrong polling venue and more or less basically the law may say u can vote but us backwards thinking white men say no so voting diminished because what's the point of showing up to vote just to be strong armed
In an effort to preserve the balance of power in Congress between slave and free states, the Missouri Compromise was passed in 1820 admitting Missouri as a slave state and Maine as a free state.
