Gee, maybe they were motivated by the horrible realization that a segment of the American population were being brutally disenfranchised simply because of their skin color.
If I have a choice between "milk of human kindness" and acquiring and holding more political power, I'm gonna assume the motive is going to be the more evil of the two.
Look up LBJ. Racist Southern Democrat turned oh so very concerned for poor blacks. What was his motivation? Political power.