A: Because 'diversity' is newspeak for Affirmative Action.
The history is simple. In the 1960s there were several social movements. Two key ones were Civil Rights and Black Power. The Black Power Movement was comprised of separatists who basically wanted more than the Civil Rights delivered. They were strident in their demands. An equal vote was not sufficient for Black Power. Dispensing with matters of causality in ways now familiar to us all, a failure in parity meant a racist agenda. Charges that wouldn't stand up in court were thus laid in the court of public opinion. In the beginning there was greatness and talent. Over time, every dude with a grievance was given a mic. Nevertheless the stark racial lines of the 60s were sufficiently incriminating so that more than mere tokenism was required. Thus the terms and conditions for Affirmative Action were broadly accepted and implemented via an executive order from the hands of Richard Nixon, whose previous policy was called 'benign neglect'.
The broad outline which characterized the 1970s was that separatism was over and integration was the new regime. Black Power melted away in substance if not in rhetoric and American institutions did their damnedest, some harder than others, to get blacks employees in and up the ranks. Crossover culture was born, blacks got into Ivy Leagues in record numbers, Richard Pryor became a superstar talking about black & white, and by 1980 we had Lando Calrissian and Lionel Ritchie. Integration and Affirmative Action were the 70s peaceful response to the riotous 60s rebellion. Of course there were overproductions and errors. But when *I* grew up, the accepted standard was that assigned in the landmark case Bakke vs UC Regents, that said race can be taken into consideration as a factor in admissions. Most specifically Bakke was interpreted as race is something, but it is not everything, and you cannot use racial quotas (which essentially makes race everything).
The political consensus emerged that so long as you don't use quotas, everybody might not be perfectly pleased with racial integration but society is so much better off that the benefits outweigh the costs. As well, people understood generally in the Bakke era, that Affirmative Action wouldn't last forever. Mathematically and socially society would run out of the cream of the crop to be integrated and like with Civil Rights and Black Power, things would go from excellent to mediocre to incompetent to worse. So long as race was not the single consideration, everything was cool.
And then there came Rodney King and OJ Simpson. Then one fateful day 'The Bell Curve' was published in a rather hasty and unsavory fashion. Essentially in the 90s, the instabilities of the 80s exploded and nobody could stomach 'Affirmative Action'. Further incremental mincing around the issue of racial integration found their way into the national dialog with more Supreme Court decisions like Hopwood. Corporations had already made their financial calculations to split the difference in hiring using the term 'diversity' rather in the same way that 'personnel' became 'human resources'.
Somewhere around the early 2000s with Grutter and Gratz being the notable SC decisions, I personally, given my own history, gave up on the premise that entrenched sides could negotiate with the sort of necessary discipline that characterized the clear-eyed hardball reality based decisions made in the Nixon Era when people had fresh experiences of cities burning. Rodney King LA Riots notwithstanding, all of black America and white America were not dealing with such weighty matters as the wholesale denial of public accommodations that characterized the conditions that generated the Civil Rights and Black Power movements. In the middle of the 20th Century the majority of American Negroes were poor. Whatever could be said about the polarizations of Rodney King and OJ, their situations did not accurately reflect what the non-monolithic 'Black Community' was experiencing. Aside from that it everything isn't just black and white. In the 21st Century, the majority of African Americans are middle class.
'Diversity' is the only thing nebulous enough to encompass the ambitions of 'people of color and women'.
These days people saying 'thats racist' or 'thats sexist' is a kind of pathetic echo of what was real 50 years ago. I am writing this answer from a community where my father could not live. In fact here's the little short story which was that my parents marched against racially restrictive covenants and my pregnant mother had to dodge a brick thrown at her from the hostile crowd. I live in that neighborhood now. My wife was head of the PTA. My daughter was student body president at the elementary school. My other daughter was head cheerleader at the high school. My son was captain of the marching band at the high school, and they all did 3.5s or better. And yet still people will tell me how racist it is over here because the neighborhood is about 1.5% black.
Still, the idea about is that 'diversity' really means 'thinking different' is bullshit. 'Diversity' means that you can show enough faces to prove your Affirmative Action program is working.
Now aside from the craptastic evasion and inflation of victimhood that the presence of 'diversity' demonstrates, the real grotesque irony is that 'diversity of opinion' is a self-defeating feature. Imagine you were on the global summit board of scientists presenting a paper on global warming, and yet suddenly you find that your demographics were not diverse enough. (Presuming there is some standard for diversity). What if you added the right mix and found that now your conclusions were not actually convergent? What if there were black science and female science and they were divergent from white male science? In other words, what is the responsibility of someone from a divergent ethnic, religious or gender background (identity) to have divergent opinions?
In short, what champions of 'social justice' really want to achieve in 'diversity' is for their special classes of people to get exactly what they presume that white males get according to some negotiated standard of identity representation. That is nothing more or less than Affirmative Action without saying 'Affirmative Action'. To say Affirmative Action instead of diversity forces champions of diversity to deal with the factual history of Affirmative Action which includes the participation of old farts like me who are old enough to remember what harsh reality racism was, even at the time of integration. Because when you include people like me, then you include my understanding of my own struggles, my parents struggles and my grandparents' struggles. And those realities reveal the utter triviality of what passes for 'racism' and 'sexism' these days. But moreover it requires a recognition of the fact that real Black Power separatists put down their radicalisms to integrate and crossover in real unity of purpose, rather than emphasize the self-serving, self-righteous, self-segregating identity politics of today.
To the extent that 'diversity' is not the deception that I describe. It's merely nice. It's pleasant. It's a new kind of first world privilege amongst the American bourgeoisie. You'll never hear any member country of the UN General Assembly stop what they're doing to hear out complaints by Americans about a lack of diversity or Affirmative Action. We're just playing with ourselves.