Comparing the Cilvil Rights Act of 1875 and 1964

If you don't really need to know the background to my question feel free to scroll down and skip the paragraph section.

After the Civil War and the destruction of slavery in this county, the national government attempted to reconstruct the American South into a more egalitarians society. One method in the development of this ideal was granting the former slaves the same civil and legal rights as was enjoyed by white society. These efforts culminated in the Civil Rights Act of 1875, which was a very forward thinking and seemingly modern civil rights law, which was frankly quite similar to the Civil Rights Act of 1964 in its scope and intent. Unfortunately due to the efforts of southern whites in resisting federal government attempts to enforce this law and other civil rights legislation, and frankly apathy by the white population in the rest of the nation as well, the Civil Rights Act of 1875 was never seriously enforced anywhere in the nation and was struck down by the United States Supreme Court as being unconstitutional except for the part of the law that pertained to African Americans serving on juries. With the demise of the Civil Rights Act of 1875 came a string of Supreme Court decisions that culminated in Plessy v. Ferguson 1896 which cemented the legal separation of the races in the private sector of the American South and other places around the nation as long as facilities for each race were equal.

Over the next sixty-eight years or so, the Supreme Court began to reverse the Plessy decision, piece by piece, until 1964 when, in that year, Congress passed the Civil Rights Act of 1964 granting the same rights to African Americans that whites enjoyed in the private sector of society while, this time, also passing U.S. Supreme Court scrutiny.

My question is, why did the efforts to provide civil rights to African-Americans fail in the post Civil War period yet succeeded in the 1960's?