As a policy, Reconstruction was always heavily dependent for its success on the sustained application of political energy. In turn, this was dependent on the support of the people of the North. So long as they supported the policy, politicians would continue to devote time and energy to making sure that it worked as effectively as possible.
In the immediate aftermath of the Civil War, this wasn't a problem. Reconstruction enjoyed wide public support and was generally seen as a proportionate response to the challenges of the post-war years. The general consensus was that Reconstruction was necessary to stop the South from rising again and to protect the civil rights of the newly emancipated slaves.
Over time, however, support for Reconstruction began to wane among Northerners. Most of them wanted to move on from the war and felt that Reconstruction was a constant reminder of it. Also, many Northerners—white Northerners, to be precise—felt that enough had already been done to ensure that the freed slaves had been granted their civil rights. There was therefore no longer any need, they felt, for Reconstruction agencies and organizations such as the Freedmen's Bureau.
It should be noted that many white Northerners, like their Southern counterparts, were white supremacists. As such, they did not believe in substantive equality—in terms of employment, housing, and land ownership—between the races. As far as Northern opinion was concerned, African Americans were only entitled to formal, legal equality and nothing more.
As Northerners lost interest in Reconstruction, politicians took note. When it became obvious to the Republicans that the policy no longer commanded wide support, they quietly abandoned it. In doing so, they effectively abandoned African Americans in the South, leaving them at the mercy of white supremacists in the state legislatures. They proceeded to reintroduce the substance of slavery if not the form, in the shape of the notorious Jim Crow laws, which would keep African Americans in a state of subjection for the better part of a hundred years.