"Lack of diversity" > not enough of different varieties . I don't like to go to that restaurant because a lack of diversity in the food they served.

"…concerned about *fair treatment for minorities."

The link to the Oscars article: http://www.theguardian.com/film/2016/jan/22/oscars-2016-charlotte-rampling-diversity-row-racist-to-white-people

You may wish to leave a question open longer in future so that you may benefit from a greater number of different answers from different members here.

I think you should learn how the word is most frequently used in today's world: "inclusion of minorities". Thus when we say that some people are worried about diversity at the Oscars, we don't mean that there is not enough variety in the movies. We mean people are concerned about treatment for minorities. Here is an article about the current diversity debate on the Oscars.