It is easy to make too much of the change in mainstream socio-politics that happened in the mid-1960s. Leftism has been increasingly mainstream in The West for a couple of hundred years - but within that project, there was a change in the 1960s; and on the surface it seemed to be a qualitative change.
Some contrasts... The Old Left was about economics, the New Left was about the sexual revolution and antiracism; equality of opportunity versus affirmative action and group preferences; nationalisation versus globalisation; planned economy versus free trade; protectionism versus mass-unlimited population migration; the native workers versus immigrants...
For a while the idea became fashionable that this was a shift from modernism to postmodernism; where modernism was seen as a kind of Enlightenment Rationalism and progress; and postmodernism was seen as relativism, loss of all values, loss of all explanations, loss of all sense of purpose and progress - a world of change but no meaning.
However, although the surface change was indeed qualitative; underneath the change from Old to New Left/ modernism to postmodernism, can be seen as an unfolding of the same underlying impulse.
How do we know this? Because many millions of individual people on The Left, en masse, made this exact transition in their beliefs; and very few of them refused to do so.
The same individuals who in their youth championed white, native-born, working class men as the oppressed 'proletarian' heroes of society... by the time they reached old age had demonised this group (as racist/ sexist/ homophobic/ Christian bigots etc.), and now champions everybody except them.
Another example is eugenics. Before 1960 pretty much All intellectual Leftists (except Roman Catholics) were ardent advocates of eugenics: i.e. of state control of human fertility as an essential means towards improving genetic quality, and therefore social functioning. By 1970, the same group of people, often the same individuals, regarded eugenics as a hallmark of 'Right Wing', 'Fascist' evil; and anyone proposing eugenics was aggressively, often violently, excluded from public discourse ('deplatformed' as people would now call it).
The same happened with 'feminism'. Before the transition; Leftists regarded feminism as subsumed within socialism (racism too) - the sexes being unified by equality of opportunity. From the 1970s, women were increasingly split from men as a victim group with opposite interests than men; and with different laws and rules applicable to women and men; so as first to equalise sex outcomes, then to make women's outcomes higher than men's - in one area of public action after another. The ideal of equality has been replaced by an ideal of inequality.
Yet although these aims (sex equality of opportunity versus sex inequality of outcome) were extremely different, almost opposite; Leftists remained Leftists - with very few exceptions they obediently followed 'the party line', and (in their multi-millions) set-aside honesty and consistency.
And Many millions more joined them - as the Left took-over all mainstream politics, government, all all major social institutions.
The fact that people on the Left so easily, seamlessly, changed - and even reversed - their superficial opinions and their policies; and did so dishonestly, claiming that they were not doing so; tells us that the roots and motivation of Leftism are not at the level of obvious opinions and policies.
The roots of Leftism are much deeper - and are, indeed, at the level of metaphysics. That is, at the level of basic assumptions concerning the nature of reality.
The basic assumptions of Leftism have unfolded over the past couple of centuries (and were foreshadowed before that time). The most fundamental assumption is a set of linked assumptions rejecting divine agency and the immaterial. These are along the lines that there is no God, and no objective truth, beauty or morality; that the material (perceptible, measurable) realm of things is the only reality; and that human emotion (pleasure-suffering) is the only valid measure of goodness (aka 'utilitarianism').
(You should note that materialism and utilitarianism are ultimately contradictory - because other-people's emotions do not exist according to materialism, being objectively unobservable, not-measurable, unquantifiable; nonetheless this combination of assumptions is universal in mainstream public discourse for the past century and more.)
On the positive side of 'what was wanted', Leftism probably began with pacifism among mid-18th century Nonconformists, the abolition of slavery spreading from this same group, and a mounting demand for relief of the new kind of poverty and misery that was caused by the industrial revolution by means of state redistribution of wealth...
And from the beginning the sexual revolution was a strong element, although initially only among the upper class radicals (e.g. Lord Byron, Shelley); who immediately used a political rationale for advocating their own practice of unbounded sexual relations outside of religious marriage.
We need to recognise that, even though its early advocates espoused some good causes, and many individuals at the low level of the movement were basically good-but-misguided people, the Left always was from its very roots a basically false (hence evil) human motivation.
By excluding or marginalising the divine perspective; by placing mortal life, materialism and human emotion as the focus of human evaluation and action; it was always inevitable that Leftism would unfold to short-termist hedonism, despair, and nihilism - and would lead its adherents (at first unconsciously, but increasingly explicitly) to seek their own self-annihilation - both in general - by working actively for the destruction of their own marriages, families, institutions and nations) - and individually.
This self-annihilation is rationalised by a publicly enforced cancerous compassion. Compassion is, objectively, a minor virtue intended as a duty in relation to a person's immediate circle of family, friends and neighbours. But post-60s Leftism has raised 'universal, unbounded compassion' to be the ultimate virtue to be striven-for - and, of course, this is a form of suicide - both at a group level and for individuals.
Self-annihilation therefore also operates personally - with its compassion-driven focus on abortion/ infanticide, and euthanasia for an expanding and open-ended scope of indications. The ideal of unbounded non-procreative sex is also justified by compassion for those with 'unconventional' desires.
Ultimately there is the increasingly-accepted/ wanted transhumanist project of destroying and replacing humans (by drugs, genetic engineering, implanted social-mass media, microchipping, downloading etc) - again, advocated mainly on the basis of compassion for suffering.
All these 'new' phenomena have their motivational roots in the centuries-old and basic assumptions of Leftism; they were implicit from its very beginnings.
Which tells us that the origin of Leftism lies in the demonic; in immortal purposive evil with foresight.
And this is why Leftism has been by far the most successful of all evil strategies in the history of Mankind.