I think a combination of COVID and tiktok, they both came up at around the same time.
COVID provided a confusing, unclear thing that was presented as this doom/danger, but then wasn't visible to the majority of people in such a way as e.g. presented in an apocalyptic movie, which meant a lot of people were confused about what the messages meant. I think it also became really clear at this point that most people struggle with nuance - they want a clear answer one way or another (the virus is incredibly dangerous, or it's harmless - the reality is it's mostly harmless to most people but incredibly dangerous if you're particularly vulnerable and/or unlucky. And larger numbers of ill people place a strain on healthcare services. But that isn't a nice easy answer.)
Conspiracy theories easily fill that gap between what people expected a pandemic to be like vs what it was actually like for the majority.
It was also a moving target - the scientific "best guess" was constantly changing and evolving, which provided more confusion. Conspiracy theories excel at giving a concrete answer to an unanswerable question. Because they thrive on being confidently concrete they like to point at scientific uncertainty and call this "lies" or "misleading".
The government did not act in an organised way, which reduced confidence. Confidence in the government was already low/fractured due to other world events (very split elections, Brexit, other "culture wars" type things, austerity, cuts to public services). People are disillusioned by all the political parties. Conspiracy theories again provide a nice neat explanation and hint at an easy packaged wrapped up solution. Real life is not like that so no real solution can exist like that, but conspiracy theorists love them because people are drawn to nice easy solutions and we want to believe they are realistic.
Tiktok exacerbates all of this by allowing anyone to make content and by having an extremely effective algorithm which quickly sorts out the engaging content from the non engaging. Meaning that the most engaging content gets out reaching billions of people, inspiring many of them to subscribe to this particular worldview, and get involved/make their own theories and videos and such.
Content that is engaging tends to be that which is extreme or provokes strong emotions in people. So you get very polarised trends on tiktok meaning that person A and person B are having totally opposite feeds, but it can feel like "everyone is saying this"
Then there are psychological phenomenon such as that if you see something repeated enough times your brain absorbs it as true information, even if you might logically know that it is not true. This is probably just a shortcut that makes sense in a biological world where you meet a small number of real humans, where faced with multiple theories/opinions, the most often repeated is probably true. It makes less sense in a digital environment where you can be bombarded with hundreds of messages within a few minutes.
Conspiracy theorists have a very clear operating formula which is to pick something that people are unclear about but which seems unpleasant in some way (whether this is government control, covid, covid restrictions, autism, letting a doctor momentarily hurt your baby by injecting them) and big up up up up the fear factor and then switch to soothing, protective, don't worry I know the answer, do you want to know? Kind of tones.
It's very very compelling. It seems nutty from the outside but they find a crack and they chisel that and exploit it. Mistrust in government is a very easy one for them. And this has become higher.
Then the next part of the formula is to pivot to the next issue. Whatever is current they jump on, to the point they become inconsistent within themselves. I was listening to an interview with an ex-antivaxxer the other day and she said that one of the big things that made her stop and think was the fact that in all her antivax groups, people had maintained that in a Measles outbreak, they would use quarantine and hygiene instead of vaccinating, but then when it came to COVID, they refused to do these things.
I think if you want to stop conspiracy theorists you want to genuinely look with compassion and curiosity, but also critically at the reasons why people lose trust in government, police, the medical profession, the media, experts in general. Most people turn to conspiracy theories because it's the first time any kind of authoritative source/voice has actually made sense to them. So why aren't the voices of the actual authorities who are responsible for them not making sense? Why aren't they finding compassion and understanding there? You often find conspiracies are rife among marginalised groups for example, who haven't been treated well by authority.