It's four factors having a unified effect. First there is a desire for moral virtue. This is populism, as in the current age the people on both the right and the left want governments to be tougher on crime. Manufacturing crime to be cracked down on or even a perception of crime that is being cracked down on is a guaranteed way for politicians to gain people's support. It's always happening in society, and with laws on sexual abuse and child abuse broadening their scope vastly and the punishments becoming harsher, this means it's ripe for picking as the holy grail of populists. "Think of the children!" is something the masses swallow every time. They want their children to be safe, including from imaginary threats.
Second, from the masses themselves there is brainwashing through bullying. If an artist draws loli, they will be bullied on social media. The smaller the artist, the more likely bullying them into renouncing their loli-lewding ways is to be successful. Once an artist gives in to the bullying, it's a silent success story: the anti-loli crowd will not make noise about it because they want it to be seen as you said, the left never tolerating it. Those who ignore the bullies and continue to draw loli get suspended from Twitter by mass reports. Big artists used to be able to protest it, maybe even get their account back, but now I believe it's explicitly against Twitter's rules to post loli drawings or to "defend child abuse". I'm not sure which site it was, but it added as a rule that posting sexual drawings of characters that "look or are said to be under 18" is against the rules and subject to banning and "if necessary" reporting to the authorities. Many sites interpretatively add this kind of a rule to show they're "dedicated to fight child abuse", because countries are changing their laws to make loli drawings illegal.
Why is it becoming a pandemic now? Because the people who push for this change have all the time they want thanks to corona. They can now pick targets and bully them in an orchestrated manner, and they have the support of most site admins due to changing laws and a desire for moral virtue. Each "victory", an artist or anybody who renounces loli drawing even silently, sends ripples through their community. Anyone asking why they no longer draw or post or support it risks getting "outed as a pedophile". From their perspective, getting you to argue "these drawings are legal" is a victory because they want to change those laws, and you're giving them evidence that they "need to be changed". They sign petitions and get governments to make such drawings illegal, and like when laws regarding CP are changed, the new laws are retroactively enforced.
Remember when real erotica under 18 was made illegal? Did law enforcement care if someone had bought a magazine or video when they were still legal? Of course not, because this isn't a legal battle and has never been. You couldn't argue "I had this magazine years before the law was changed", you had to throw it away or you were at risk of prison. This created a black market for such magazines and videos, which operated on the streets for years until law enforcement had raided the supply of such magazines and made arrests on those who bought them. They remain in private possession, continue to be sold in the black market and sometimes even out in the open online. It will be the same with drawings, considering one big difference. It's much easier to draw a high school girl posing nude than to find a high school girl to pose nude and photograph. This is doubled with porn, tripled with fetishes...
Lewd drawings shouldn't be on the same spectrum as erotic photography, and erotic photography as in such magazines shouldn't be on the same spectrum as CP. But, like such erotic photography came to universally be seen as on the same spectrum as CP, so will lewd drawings.
Third, connected to both the above is the left's desire to show that we are better than the right. It gets more visible as we are losing the battle for the masses globally, so often we lose another talking point to the right and the masses latch onto it as a right-wing issue even if it is not. We want to appeal to the masses, so we then latch onto the same populist talking points used by the right but approaching from a left-wing side. On this topic, it's an issue of "moral virtue" for both the left and right: for the right because they consider it morally wrong and for the left because we see ourselves as having to be morally superior to the right.
Fourth, the left is regressing. The term "regressive left" is a meme but there's a degree of truth to it, however relevant to this topic is only that there is an increase in nostalgia for previous leftist movements instead of looking forward. We look at history and think what could have been if only a revolution had succeeded or if a socialist country had remained socialist and progressed from there instead of under capitalism. This isn't harmful by itself but if we start to think that we should reject modern progress in socialism, it creates the situation where solutions are sought from elsewhere or from the past almost religiously. Looking at the laws of other or past socialist countries for guidance instead of thinking for yourself is no different from looking for rules in religious texts for guidance.
With time people become less patient, attention spans get shorter, and no one wants to think for themselves. Even on the left. We have to think for ourselves and encourage others to do the same, and not just refer to books for them to read. They probably will not read them, and even if they do, more likely they will adopt ideas and talking points without even thinking why they want to adopt them. It's easier to repeat a quote or tell someone to read something than to say it in your own words, but the idea that something is only worthwhile if it was said by someone "important" should not belong on the left. Marx wasn't born the father of modern socialism, he grew up to become the Marx the world came to know.