That Speech was given in 1963. King died in 1968 and much occurred between those two dates. General economic inequality of the oppressed was recognized. Focus also shifted to the war in Vietnam. Note, however, that as a biological concept, the term race is almost meaningless.
Racial categories are human creations.
The intense sociological interest in race is due to the fact that people attack meaning and values, either real or imaginary, to physical differences between groups of people . These would include a common language, religion, national origin, dietary practices , etc. Culture is learned . A minority is a category of people who lack power, privilege, and prestige in social, political or economic spheres . Minorities must always be understood in relation to others in the social structure.