The shortest and simplest answer is the rise of the Left in its influence on the mainstream over that time, in academia (most universities), in the cultural elite (arts, films, etc.), and increasingly, in mainstream politics and the media. These all feed on each other.

First and foremost, let me make clear that there is no "leftist conspiracy." There is no organized movement, or secret Marxist cabal doing any of this. There is no dire force "behind it all," or any single source of these views and attitudes.

It's just the general environment having its effect on the people in it.

The closest thing I can find that explains the effectiveness of this movement is "osmosis," or "peer pressure."

Much of this starts with our institutes of higher learning.

Academia (in general, not universally) was coopted by the Left long before anything else. The universities are almost like a nature preserve for leftists and Marxists who can't survive in the outside world. The bad news is that they are also teachers of our young generations. This adds up over time.

It is here that young people learn that crapping where you live is the highest form of intellectual sophistication, and where things like "flag waver" become the worst of insults. It's where you learn that America (and often Israel), capitalism, our military, and our influence on the world are the source of the world's problems, and where the U.N, the Palestinians, Castro, Eurosocialism, and all the rest are raised on a pedestal.

I know from personal experience what it's like to be a conservative/libertarian in a college classroom. And I was in a semi-technical field. God help those in the liberal arts.

As for culture, the bitter opposition to what might be called "mainstream values" by the influential in the art world is self-evident. I won't go into modern art here. It's too easy.

I have heard from conservatives in Hollywood who shut up to get along. Some stars are secure enough to make their beliefs known, but they are few and far between. If you are on the lower tiers, you keep quiet if you don't toe the party line.

The Democratic Party of 1960 is most definitely not the Democratic Party of today, something that seems to escape those who continuously use JFK, Truman, and others as examples of Democrats with firm stands on foreign policy. I can just imagine what someone like Truman would make of the current Democratic leadership and its views.

The media, whose members are largely products of our academic environment, are liberal by an enormous margin. I have more than one survey showing this. Again, this is not the result of any organized movement, or "talking points" broadcast to the press by a secret cabal, but simply the result of journalists swimming in an environment where liberalism is "normal," and conservatism is cause for raised eyebrows, at best. That the media cuts the Democrats a hell of a lot more slack than Republicans is not "the fix being in," but simply what they consider "normal" versus "outlandish." They like Democrats more, and want them to succeed, and even when they try to be evenhanded, (many don't even try), it leaks out from between every line.

This same kind of peer pressure is moving into the sciences as well, which is not good.

If you want it broken down to the basics, in Democratic politics, the media, academia, and the cultural elite, to hold, or claim to hold, liberal or leftist views on different subjects means being "one of the gang." A "normal guy." To be anything other means swimming against the stream. In some of these fields, it can even have an effect on your career.

That's how this attitude grew over time. With some noted exceptions, particularly in politics (the Reagan years come to mind) or culture (films with family values still usually hugely outgross the films that delight viewers at Cannes or Sundance) the slow flow in this direction has been largely unstopped over the past thirty or so years.

This not very well organized, but maybe it helps. You're relatively new here, so you don't realize that I have said things like this often enough that simply bringing it up usually earns me a fusillade of verbal tomatoes. But you asked.