Shelby Steele offers a compelling explanation.
In the '60s, the Left acquired its power and moral authority when it fought the good fight against racism and segregation and for civil rights. Those battles were fought and they were won. But power is intoxicating and those who came into it in those years of ferment desired to hold on to it and expand it. The power proved to be not only intoxicating but corrupting. Nothing new, of course: power tends to corrupt, absolute power . . . . You know the Lord Acton riff.
To maintain their power, leftists needed to find additional sources of menace to the nation's moral legitimacy. Steele (emphases added):
The greater the menace to the nation’s moral legitimacy, the more power redounded to the left. And the ’60s handed the left a laundry list of menaces to be defeated. If racism was necessarily at the top of the list, it was quickly followed by a litany of bigotries ending in “ism” and “phobia.”
The left had important achievements. It did rescue America from an unsustainable moral illegitimacy. It also established the great menace of racism as America’s most intolerable disgrace. But the left’s success has plunged it into its greatest crisis since the ’60s. The Achilles’ heel of the left has been its dependence on menace for power. Think of all the things it can ask for in the name of fighting menaces like “systemic racism” and “structural inequality.” But what happens when the evils that menace us begin to fade, and then keep fading?
It is undeniable that America has achieved since the ’60s one of the greatest moral evolutions ever. That is a profound problem for the left, whose existence is threatened by the diminishment of racial oppression. The left’s unspoken terror is that racism is no longer menacing enough to support its own power. The great crisis for the left today—the source of its angst and hatefulness—is its own encroaching obsolescence. Today the left looks to be slowly dying from lack of racial menace.
[. . .]
Today’s left lacks worthy menaces to fight. It is driven to find a replacement for racism, some sweeping historical wrongdoing that morally empowers those who oppose it. (Climate change?) Failing this, only hatred is left.
Hatred is a transformative power. It can make the innocuous into the menacing. So it has become a weapon of choice. The left has used hate to transform President Trump into a symbol of the new racism, not a flawed president but a systemic evil. And he must be opposed as one opposes racism, with a scorched-earth absolutism.
[. . .]
Yet the left is still stalked by obsolescence. There is simply not enough menace to service its demands for power. The voices that speak for the left have never been less convincing. It is hard for people to see the menace that drives millionaire football players to kneel before the flag. And then there is the failure of virtually every program the left has ever espoused—welfare, public housing, school busing, affirmative action, diversity programs, and so on.
For the American left today, the indulgence in hate is a death rattle.