why were African Americans still not free even though President Lincoln effectively ended slavery 100 years prior

African Americans were still segregated from White people and did not have the same rights.

Explanation:

Even after slavery ended, African Americans were segregated (schools, bathrooms, water fountains, buses, etc.). They also were not allowed to vote until the Voting Rights Act of 1965. Discrimination against African Americans was allowed and continued to happen until the Civil Rights Act of 1964.

Other reasons: After slavery ended, many African Americans were unable to find jobs after they were set free. This led them into sharecropping, which was effectively like slavery. They could live as tenants on a landowners land to farm, and the landowner then took a share of these crops. Most of the time, African Americans were forced to extensively work to farm, and had most of their crops taken away by landowners afterward, so they were making little income.

they change the constitution.