See also: What the fuck does praxis even mean?
In my opinion, I think it's really kind of for three interlocking reasons.
The first is that The Left has increasingly become the politics of /ressentiment, with very few exceptions. This is not inherent to many of the projects of the Left, but it's inescapable in Leftist culture as a whole. "Leftism" is a system of morality and values constructed by the weak, the oppressed, the disabled, and the marginalized, in order to define themselves as good in opposition to those that hurt them. It's a negative value system and a negative set of ideas that defines itself specifically as antithetical to the values of excellence, power, strength, skill, the will to power, self overcoming, self enhancement, and so on, because those are associated with the morality and capabilities of the oppressor class.
This creates two overlapping problems: first, it self-selects to be popular among people who don't have power and fundamentally can't become powerful, or, if they did become powerful, would act spitefully, self destructively, and against human excellence. Second, it also directs people's minds down pathways that lead away from them having any power to do anything: if the will to power is wrong, if excellence and strength are wrong, if power is to be avoided, how can you achieve anything? All you'll achieve are endless meetings, begging for the approval of others, and Tall Poppy syndrome.
This can be fixed, through post-leftism.
Through creating a politics of radical elitist egalitarianism — believing that everyone is capable of becoming strong, powerful, great, interesting, and extending their will to power, and that the reason we are not all such supermen already is an anemic culture and economy caused by conservatism, fascism, and capitalism, and seeking to create, through mutual aid associations and communities, a world where everyone is best able to achieve and extend that will to power without hinderances from contingent things like neurodiversity, disability, poverty, etc — where such will to power is only subject to the game-theoretic constraints of Shawn Wilbur's notion of the "anarchic encounter" (a synthesization of Proudhon and Stirner that recognizes that when everyone's will to power is equally maximized as much as possible, if two wills conflict, the solution must always be mutual and 50/50, otherwise the system becomes unstable) so that, as Benjamin Tucker says, "Every man is free to do that which he wills, provided he infringes not the equal liberty of any other man." Not as a moral principle, but as a principle embraced by the politics as the necessary prerequisite for us all to get as much of what we want as possible.
Through creating a politics of benevolence and community that comes from a place of confidence and strength and joy that overflows so much that you can't help but share it, and a Sitrnerite pleasure and love of the company of other specific human beings for their own sake — not superiority or noblesse oblige, but also not weakness, fear, or white/rich guilt, or a belief that you owe communities anything.
Through creating a politics that does want to "go gently into that good night" by admitting that a free, equal, and autonomous society will be poorer, less innovative, and more akin to degrowth and primitivism, but is able and willing to harness postcapitalist desire and semiotic production, as Mark Fisher and others have theorized.
Through a politics that does not fear alienation or commodification, but learns how to use it to create freer cultures, as xenofeminism does; that knows how to use the incredible information processing, organization, and autonomy generating abilities of markets — when divorced from capitalist property norms, and based on occupancy and use plus usufruct instead.
The seecond problem is that although right-wing ideologies are often couched in terms of purity and morality, under the hood all they care about is raw power and preserving privige. That's it. They'll do or say anything, work with anyone, and forgive anything, if it serves that end. It is only really left-wing ideologies that actually seem to deeply care about being moral — or at least, the appearance of being moral… In any case, this leads to all kinds of purity testing and factionalism, and further reinforces the "politics of ressentiment" aspect.
The only way to solve this is to create a post-leftism that completely does away with moral appeals at all. That recognizes that leftist projects are merely the will to power of the disspossesed — that we want these things because they benefit us, and that's it. So we work toward our own benefit, just as the right wing does; the only difference is that we conceive of our benefit more broadly and we have different interests.
The third problem is that while right wing ideologies favor the status quo, while left-wing ideologies favor change. Thus, entrenched hierarchies of power, those that already have access to all of the decision-making processes, the resources, the cultural power, and the manpower, of established society, are always going to tend right wing, while any elements of soceity that are out of power will tend to lean left wing. This creates a similar problem as the above: those that have power will never be left wing, and worse, if left-wingers gain power, they will become right wing again, inevitably. Scratch a revolutionary and you get a reactionary.
We can solve the first half of this problem: by accelerating deterritorialization, the every-faster breakdown of traditions, obsoleting of old hierarchies and stable categories, the creation of new modes of production, communication, and self enhancement, we can destabalize where real power lies, leaving it up for grabs by those who don't have the old notion of power. If we are ready to sieze that destabalization, through the development of decentralized systems of association, autonomy, mutual aid — i.e., dual power — prior to and during it, we can stand to benefit the most from it. This is the only way the left could gain power. We haven't been very successful at it so far — just look at the reterritorialization of the internet since the turn of the century — but that's often because of the problems I previously mentioned, or a naive belief in the inevitablility of our victory (the internet was captured because people thought it was inherently free and good, and so didn't organize enough to protect it). And this is our only opportunity. But it also means that we have to take part in the deterritorialization, instead of constantly just trying to restore old things we felt comfortable and safe with.
As for the second problem, I genuinely don't know how to solve that.