Why did congress reconstruction efforts to ensure equal rights to the freedmen fail?

Well not everyone believed that the Laws of Congress were equal enough for their needs Down south some people instated "black codes" that suppresed the blacks almost as much as slavery. So they formed white supremisses groups like the ku klux klan that terrorized freedmen from voting

and the federal government was forced to withdrawl it's troops from the south so there was nothing that could be done to stop it.