I came across this thread which had some interesting interpretations and posits that ML grew as a function of anarchism's failure to recruit/organize:
My sincere answer for why tankies reemerged five years ago is that movements are social hierarchies and newbie teens don't want to compete for status in existing illegible/inaccessible spaces like anarchism, so they resurrected a dead/empty scene that had trappings of status.
See also leftypol & the dirtbaggers. Folks get converted on one issue and then recoil about being expected to also learn / change their opinion on a variety of other topics. Respecting pronouns?! Never! You olds are a joke! We're making a new movement with hookers & blackjack!
Most of the anarchist movement had sneered at and avoided the internet (seen as an insecure tool of civilized alienation). Also it was illegible, most of the shit we expect you to learn/accept we don't even write down. And getting involved? We're terrible at helping folks join.
But ALSO the anarchist movement got up its own ass. We derided the internet and avoided utilizing it effectively. We embraced illegibility as resistance, forgetting that accessibility is critical to undermining hierarchies. And we corrupted into playing internal status games.
So what do we ascribe the sudden uptick in radicalization?
I think a lot of people who call themselves "MLs" really just read some Marx and Lenin and found them to be insightful, rather than subscribing to the state ideology of the Soviet Union, or organizing around doctrinaire ML principles. If Marxism-Leninism were truly on a meteoric rise, we would have a communist party at least the size of DSA, but we don't.
The reason Marx and Lenin are rising in popularity are because their works are useful and explanatory. They help people understand what is happening in the world around them, while a lot of (but certainly not all) Anarchist writing tends to be more utopian and idealistic - explaining what the world may look like if everyone treated each other as comrades and shunned power imbalances on principle.
In the US, I think the Anarchists are also hindered by the rampant individualism which is baked into our culture. I don't think this is the fault of Anarchist theory, but at some level anti-hierarchical practice needs to take on a collectivist form to have any chance at long term success. This is a difficult point to impress upon newcomers without some sort of official political education program. When you shift from a critique of hierarchy to the practical details of navigating a world full of hierarchies, there is a lot of nuance to work through and opinions are very fractured.