Why did Black Americans fail to receive full freedom after the end of the Civil War in 1865?
I would argue that this failed to happen because there were not enough whites (in the North or the South) who were actually dedicated to the idea of "full" freedom for blacks or for the idea (which may be the same as "full freedom") that blacks deserve to be equal to whites in the eyes of the law.
Not surprisingly, much of the South wanted to retain white supremacy. This had been a cornerstone of their whole social and economic system and the loss of the war could not easily change it. Blacks' only hope, then, was that the North would force the South to accept full freedom for African Americans. But this did not happen because whites in the North were not sufficiently wedded to the idea of black equality. They were not willing to maintain the anti-democratic aspects of Reconstruction simply in order to bring about racial equality.
Thus, overt racism in the South and a lack of strong sentiment for equality in the North made it impossible for blacks to receive full freedom in 1865 or for decades after that.