The question is an interesting one. The most evident answer would be that the Constitution failed to guarantee rights for everyone when it was originally written. Its original ratification in the 17th Century failed to effectively address the rights of African- Americans, women, and young people. In terms of African- Americans, perhaps this was because the issue of being a person of color in America was something that the framers simply did not understand. It was evident that many of the framers failed to understand the political and moral wrong of slavery. The nuances and implications of race in America were simply beyond the comprehension of the framers. It is in this light where I think that the Civil Rights Movement was needed.
The Civil Rights Movement desired to make a social, economic, and political statement about what it meant to be African- American. The movement's focus on race as a critical aspect of consciousness in the modern setting helped to initiate a discussion that had not been prevalent on a national scale. The Civil Rights Movement was needed to start the change in thinking about race in America. It was needed for both people of color and White Americans. Both groups had to reflect and think, along with having some fairly uncomfortable conversations about race in America. Even with the addition of the 13th, 14th, and 15th Amendments to the Constitution, this dialogue was sorely needed. The issue of race in America, why it was so prevalent to millions of Americans, and how race is perceived were elements that had to be raised. The Civil Rights Movement recognized that this discussion was needed in as many realms of American life as possible. It brought out a reality that theoretical freedom is fundamentally different than a practical sense of it. The Constitution's promises and possibilities could only be fully revealed with a national discussion in as many realms as possible about what it meant to be free and to be a person of color in America. It is in this light where the Civil Rights Movement was needed in America.