The left used to be more individualist in the U.S. (circa 90s most definitely) but it developed a toxic groupthink as it came to dominate pop culture and media in the 00s and 10s, and began to leverage that to employ censorship, deplatforming, doxxing, etc and it became incredibly dogmatic and if anyone diverged from a particular narrative (ie skeptical covid came from wet market), they would be ridiculed, shouted down, laughed off, shamed, kicked off social media platforms, ostracized, etc which is cult like behavior.
The left of the 90s would have never stood for that. They were the die hards for free speech then. Something shifted.
Yeah, the left of the 90s would never excommunicate anyone for being completely certain Iraq had weapons of mass destruction, and that climate change was a hoax, and the moon was made of cheese, and bringing these things up at every opportunity, because the left of the 90s believed in free speech.
Individualism has been failing Americans, while the quality of life has improved dramatically in less individualist cultures, in many ways surpassing Americans (health care, housing, education, upward mobility, etc), so it shouldn't be surprising that collectivism is starting to win mindshare.
Don't fool yourself, conspiracy theories are usually marginalized in US culture, the left didn't welcome conspiracy theories back then either.
Also, now that political correctness and censorship mostly coming from the right again (with the Moral Majority going after music then, and video games/porn now), we now see the civil libertarian elements of the left standing up to fight censorship once again.
The boomers got old. They were flawed but the heavy anti-authoritarian vibe of the old left was largely carried over from the 60s. Imagine a zoomer lefty burning their covid card the way hippies burned their draft cards. Nope! The thing that shifted was authoritarianism; the terminal end-stage of socialist ideology.
The left of the 90s would have never stood for that. They were the die hards for free speech then. Something shifted.