This is a fantastic question! Traditionally, the "Civil Rights Era" encompasses 1954-1968, from the legal case Brown v. Board of Education striking down Plessy v. Ferguson all the way up to April 1968 and the Assassination of Martin Luther King. Its important to note the three Civil Rights Acts in the Civil Rights Era: 1964, 1965 and 1968 which destroyed Jim Crow occurred during this era as well, desegregation and equalization was occurring nationwide to many different degrees.

After King's death, the texture of the Movement changed. There was a split in how to take things on afterwards, continuing with pacifist and legal methods or more revolutionary actions a la the Black Panther Party. In addition to this, the 'Enemy' of Civil Rights, Jim Crow, was largely destroyed. Its easy to fight to take down "Colored Only" signs, its significantly harder to attack poverty, the prison industrial system, inequality in the workplace, personal racism and so on. This lack of defined 'enemy' really hurt Civil Rights Organizations after the Era up to this day. Another possible reason, and I may just be speculating, is that the Movement after 1968 lacks truly identifiable leaders, everyone knows Frederick Douglass, everyone knows DuBois or Washington, everyone knows King or Malcolm X. After 1968 its much harder to find a leader that identifies with the Movement as a whole.