Some people think it's because they all got killed.
But I think it has more to do with the decimation of the manufacturing base in the urban centers. Senator, an optimistic, energized population throws up optimistic, energized leaders, and when you shift manufacturing to the sun belt in the third world, you destroy the blue-collar core of the black activist population. That is what energized the community for the Civil Rights movement of the 50's and the 60's. An energized, hopeful community will not only produce leaders, but more importantly, it'll produce leaders they'll respond to.