Why did Radical Reconstruction in America ultimately fail ?
This is, to some degree, a matter of opinion. My own view is that Radical Reconstruction failed because not enough white Americans, whether they lived in the North or the South, cared enough about the rights of African Americans.
Radical Reconstruction came about for two main reasons. First, the Radical Republicans were very angry with the South because of the Civil War. Second, they were angry because the Southerners were mistreating African Americans. Over time, anger at the South because of the Civil War waned. The issue, then, was whether Americans would care enough about black rights to continue Radical Reconstruction. By the mid-1870s, the only point of Radical Reconstruction was to ensure that Southern states respected the rights of their black citizens. If whites did not care about this, there was no further reason to keep Reconstruction going.
By 1876, even many Northerners were ready for Reconstruction to end. They felt it was antidemocratic because it did not let Southerners really govern themselves. Importantly, they also did not care very much about black rights. Very few white Americans in the 1870s truly felt black rights was an important issue. Very few white Americans felt that African Americans were truly equal to them. Therefore, they were not interested in maintaining an undemocratic system of Reconstruction simply to help protect black rights.
Because very few white Americans cared very much about the rights of African Americans, Radical Reconstruction ended in 1877. They were willing to let Radical Reconstruction end even though it had failed to do enough to secure the rights of the newly freed slaves.