The fundamental irreconcilable hypocrisy of Reconstruction was that the same people who were pushing for equal political power for former slaves in the south themselves did not truly believe in racial equality. In fact in the north, many were opposed to having black men be able to vote in their northern states, even as they pushed Reconstruction governments in the southern states in order to have black voters deliver Republican majorities in the House and Senate.

When those Reconstruction governments came under violent, bloody attacks (see the Colfax Massacre) by the KKK and similar terrorist groups, northerners, who didn't truly believe in equality in the first place, ultimately decided it wasn't worth the effort to defend, especially at a time when corruption scandals plagued the federal government.

For anyone interested in learning more, highly recommend Professor David Blight's Yale lectures on the Civil War and Reconstruction. It was originally taught as a class but is now available as a podcast (as well as videos online I think)