Why is the Reconstruction era considered a revolution, and why does Foner say it is unfinished?

The Reconstruction Era began a revolution in the freedom of former slaves. They experienced new elements of freedom, such as the rights of citizenship given in the 14th Amendment. African-American men were given the right to vote with the 15th Amendment. In addition, at least in theory, they were freer to control their own movements, marry, and join the church and community organizations of their choosing.
On the other hand, the reality of Reconstruction was that the promise of complete freedom for former slaves was not fulfilled and the revolution, as Foner writes, was unfinished. For example, while some freed slaves were promised land, not many were able to attain land ownership and instead worked as sharecroppers on others' land. In addition, the right of African-Americans to vote was often curtailed in the later years of Reconstruction and afterward. The revolution that began during Reconstruction continued into the 20th century and beyond.