This is an interesting question - and the supposed answers I have encountered are unsatisfactory.
The fact that needs to be explained is that the metaphysics, that is the fundamental foundational beliefs of 'Leftism' are not only incoherent (which they are) but incoherent in a way that leads the system to consume-itself.
This self-destroying quality of Leftist metaphysics is apparent in
the history of the political Left, and it is also apparent in the
personal history of many Left-adherents.
(Note: Leftism here includes not just communism, socialism, nationalism, national socialism, liberalism etc - but also all mainstream forms of conservatism, republicanism, libertarianism etc. The true polarity is between Leftism and Religion; and the commonly-asserted Left-Right polarity is interpreted here as merely different varieties of secularism, therefore different varieties of Leftism. In other words, the only way of being non-Left is to be religious. Various religions are possible. To be non-Left is necessarily to advocate that the state be primarily religious with all other activities and functions subordinated to religion.)
The temptation, which leads nowhere, is to try and construct a Leftist metaphysical system from some specific attributes - and in contrast with the pre-ceding and rival metaphysical systems of religion (different systems for various religions and denominations).
But this doesn't work, because the Left keeps changing. At one time it was plausible to assert that the Left was about equality, but that clearly is not the situation now. At another time - say 30-40 years ago - it was suggested that the Left was focused on reducing suffering and humiliation - but again that can now be seen to be merely a temporary phase.
And an historical consideration shows that the root of Leftism is not some positive doctrine, but a negative subtraction.
If we suppose that natural, spontaneous human metaphysics is religious and (sufficiently for life) coherent and systematic - then Leftism comes into the life of a person or a culture by subtracting something from this religious metaphysics. What might that be?
The first suggestion might be God or the gods - but I don't think that is necessarily correct: God/s may be allowed to retain their place (for a while) but certainly the definition and scope of God/s is the thing that is change.
I think the specific change is related to the subtraction of purpose - or to 'teleology'. Leftism removes ultimate, objective purpose from the metaphysical system.
In consequence, because we cannot live without purpose, instead of a unity and reality of purpose we get several or many purposes, that are not unified, and are not objectively real or relevant and compelling.
The earliest defined form of Leftism, setting itself up as rival to religion, was Marxism - and in that early Leftism there was a kind-of residual purpose that was was taken from the idealistic philosophy of Hegel in the form of an abstraction related to the direction of history. History had an inevitability of progression - and if someone wanted to be on the same side as history - that is, the 'winning side' - then he should be a communist. However, there was a major hole in the system in that there was no ultimate reason why it was good or necessary to be on the winning side - especially in the short term it may well be 'better' (yielding more pleasure and/or less suffering) for someone personally to oppose history.
So, even in its earliest and most 'religious' phase, Leftism did not provide an individual purpose - although it did provide a goal and direction.
It seems to be that it is this subtraction of purpose from our underlying metaphysical system characterizes Leftism - but the question arises of why anybody, or any culture or civilization, would want to subtract purpose from their foundational beliefs?
Since we have, most of us, experienced this for ourselves at some point in our lives, I think the answer is available to introspection (after which is can be checked against experience and evidence) - and that answer is liberation, freedom, escape from aspects of purpose that we find thwarting, oppressive, or in some way aversive.
This is the reason why sexual revolution is so often integral to Leftism - a fact difficult to explain otherwise. Because there are few people who do not feel, in some way and to some significant extent, constrained by the sexual rules and exclusions of religion. Likewise, people feel constrained by their social position, class or status; by actual or relative material insufficiency; by their nation of origin or residence; by their appearance or by some deficit... there are many possible reasons.
Leftism offers a liberation from the necessity of such constraints and others by its removal of purpose from ultimate understanding - there can be no ultimate reason for constraint if there is no ultimate and unifying purpose: so that particular problem is solved, whatever the problem may be. Constraint is removed, or else there is hope for this - and there is liberation.
Thus Leftism has a universal message and a universal appeal.
Whatever your personal grievance against Life, Leftism offers actual or potential liberation from it by means of the removal of ultimate purpose and removal of our ultimate obligations to that purpose.
And this seems to explain why Leftism is always unstable, always moves from one liberation/ destruction to another, is always - sooner or later - hostile to any religion; and always ends-up in approaching nihilism.
Because nihilism (which is the conviction that nothing is really-real) is necessarily the end-point of any world view that lacks 1. ultimate purpose and 2. an ultimate rationale for each person to subscribe to that purpose.