This is where I wanted to go with this topic from the start.
Through the mid-60's, becoming a mainstream American was the fashion. In none of the civil rights rhetoric from Reconstruction to Selma was there the least blather about diversity being our strength. MLK's speeches hew close to the theme of the black man being counted in the mainstream, of coming into the mainstream, of being deserving and eligible of being in the mainstream.
Neither was there much celebration of differences among other ethnic groups. Every group had its customs, and it was considered acceptable to cherish those. But it was also considered acceptable to abandon them completely, and rejoice solely in having "made it," being accepted as real Americans. Greeks, Irish, Hispanics - we tended to call those "Latins" in those days - measured their progress by mainstream acceptance.
While there were some voices early on in feminist writing suggesting a restructuring of roles and society entirely, most of the early emphasis was on "women are as good as men, the same as men, should be treated the same as men, etc." WASP culture was not despised, it was envied, admired, and imitated. It was the fashion.
That is no longer the case. To be different from the mainstream - not hugely, but distinguishably - is now the fashion. I don't say that one fashion is better than the other. the change worked against Mr. Romney and for Mr. Obama in the last election, which I regret, but I don't see that either fashion is better. (There is some exception to this, which I may get to. But indifference is my starting point.) I don't say it's worse that difference is now the fashion, but I note that this is so. We used to brag about being a melting pot. Now we prefer to say we are a mosaic. The religious drive in the 1960's was to be "ecumenical."
Offer whatever theories you like about how this occurred: the changes in immigration policy in the 1920's and the 1960's; the purely visually imaginative difficulty of incorporating black faces as representative, which had not been so hard with Slavs, Jews, Swedes; the shattering of the idea of unity during the Protestant Reformation, or the discrediting of ideas of unity by the National Socialists; increased worldwide communication or travel; communist propaganda to divide Americans; watching TV; drugs and novelty-seeking; Tolkien and Lewis lauding the diversity of fictional other sentient creatures. Whatever. It is enough to notice that the previous ideal is now an object of scorn. Everything from "My Big Fat Greek Wedding" to performance art does not merely exalt the hipness of rebellion, but the contemptibleness of being "whitebread," boring, married, straight, 9-to-5, parents. Much better to be "quirky," eh? And it has long been a cliche to note that the art community values what is "transgressive."
Is this some revenge factor, or a sour-grapes response from individuals who couldn't cut it, and projected their own rebellion onto their group? Is it just the natural flow of trendiness? Dunno.
In evolution, both values much be present for survival, of course, and they have always been held in tension. One must be conformist enough to remain in the tribe, but different enough to attract a mate. These themes have likely played out in every century, and we are merely seeing one aspect at present. We did celebrate differences even in the bad old days, and plenty of people still aspire to the mainstream now, caring not a fig for diversity. Or they aspire to a new, different mainstream, just as conformist as the old one but with a beat the kids can dance to. Though there is reason to think this may be a bit new. America is far more a mixed society than any previous one. That's not easy, and perhaps embracing a new extreme of celebrating diversity is the only way of getting along, even if a lot of it is strained and mere formality.
I make no prediction where this is going, nor what outcomes are to be preferred. I have ideas, but I haven't really played them out in the imagination fifty years or so.