The short and incredibly stupid answer: Gamergate.

The longer and equally stupid answer: the way that gamergate and its ancillary movements created a sort of fork in the road for the sort of disaffected chauvinist libertines that broadly dominated American counterculture before that point, making them choose between either reforming and bettering themselves as a person or fully embracing all their most toxic and self-serving desires and going full fash.

The longest and most rambling answer:

A major problem that the left has had standing in its way in the US has been just how entrenched patriarchal and white supremacist ideals are in the American conscience: this was a massive hindrance to organized labor in the late 19th and early 20th centuries, as workers themselves often harbored toxic white supremacist beliefs that made solidarity impossible for them; this was a problem for the anti-war movements of the 60s, where many of the people passionately decrying the war in Vietnam were more concerned with not dying there themselves than with establishing any sort of justice or equality and the "hippy" movement itself included virulently racist and sexist contingents; this was a problem for the post-cold-war left, where socialism was in full retreat and chauvinist libertines fully took over the vacuum of dissent against the far-right status quo, as they were generally more privileged straight white men who simply wanted to be personally free from the restrictions that both theocrats and liberals alike believed in and supported.

Gamergate was the crystalizing moment that, to put it bluntly, created a space on the far-right for those disaffected chauvinist libertines and gave them a framework for viewing the world that despite being completely incoherent and vile made them feel like they were part of something, like they could all be the sort of heroic warrior they'd wank themselves dry fantasizing about being, and which directed their hatred towards the left and its gradually increasing resolve against chauvinism and predatory abuse.

Similarly, it was a crystalizing moment for the left as well, since it drew up clear lines of conflict between reactionaries and decent people instead of just letting the American left stew as one big morass of general dislike for theocracy, warmongering, and austerity. Like I remember pre-gamergate reading a paper Matt Taibi owned called The Buffalo Beast, and it really encapsulated what I'm talking about it: it was vulgar, only had the most barely coherent of succdem politics, and continuously flirted with reactionary, chauvinist ideas including its editor going on an unhinged transphobic screed defending his use of slurs. That's the most coherent image of the American left I have from back then, that and the weird sex-pests that Richard Stallman platformed on his blog. Gamergate took most of the vilest elements of the sort of left-by-default counterculture for the fascists, and what was left well, that was forced to reform and become more coherent, leading to a resurgence of socialist thought and the rise of intersectional feminism on the left.

Now, is that schism complete, with all the chauvinists either gone or reformed? No, obviously not, as we can see with stupidpollers and other reactionary sucdem movements, but it's a hell of a lot clearer than it used to be.

Extra note: The Buffalo Beast itself once posed the question "What happened to the 'South Park Republicans'?" and concluded "they became democrats and support Obama," and like, meant it positively. It kind of reminds me of an editorial in an issue of Heavy Metal from 1980 doing apologetics for Reagan as "the less conservative candidate" compared to Carter, in that both articles hit the same sort of notes of dramatic irony.