Why did Reconstruction fail?
I would say that Reconstruction failed for two main reasons. It failed because white people in the South generally hated it and because white people in the North did not care enough to continue to enforce it.
Most white people in the South hated Reconstruction. They felt that it was being forced upon them undemocratically by the North. Because of this, they resisted it vigorously. Some white Southerners resorted to violence and terrorism (this was the beginning of the KKK) to resist Reconstruction. This meant that it was hard to keep Reconstruction going.
If the North had really cared, Reconstruction might have continued. But most white Northerners did not care about Reconstruction by the mid-1870s. They did not really care whether African Americans had rights now that they were no longer enslaved. They were certainly not very interested in continuing to impose Reconstruction on the South just so that African Americans could have civil rights.
Thus, Reconstruction ended because of opposition (often violent) from white Southerners and because white Northerners did not care enough about black rights to continue to support Reconstruction.