There were at least two reasons for this.
First, the laws did not do anything to promote actual economic equality. Many blacks had hoped that the Civil Rights Movement would end with blacks and whites actually having the same opportunities and being roughly equal in economic terms. The fact that Martin Luther King, Jr. believed this can be seen in the fact that he changed his focus to economic issues after 1965. He was killed while in Memphis because of a strike by garbage collectors.
Second, the laws did not addess the sort of de facto segregation that occurred in the North. It did not cause integration of housing nor did it cause cities like Chicago (where King led protests) to do much to integrate their work force. This was particularly important in the North where schools were effectively segregated not by law but by the fact that African Americans were forced (again not by law) to live in areas separate from whites.