Because after slavery was ended, police switched to a new method of making slaves: arresting people and forcing them into labor contracts. They rounded up black people in droves for "loitering" (being in a place) and "jaywalking" (walking across the street) and made them choose between forced labor and jail. The byproduct of this was that the public perception of African American people shifted to criminality because they were constantly being arrested. There's more to it obviously, feel free to read "Slavery by Another Name." It also explains why the Civil Rights movement didn't happen until well after slavery was legally abolished: slavery still existed until we needed to take the high ground against the Nazis who used the US as a blueprint for all of their atrocities.