Why do speakesr refer to martin luther king and malcolm x so much in speeches?

The 2 people you mention were great orators, the speeches they made held an importance to many during the United States troubled past, racism was rife and these men put themselves forward to speak out even though they knew that their lives were in danger. What they said was our inheritance, we look back with sadness and with gratitude that they left us their words. It is no surprise that people quote the words of these heroic individuals, because their words will live on and are worth repetition.