Model sites featuring underage girls showing the world that they are, contrary to the popular narrative, attractive to adults and thus capable of making big money are going to be targets of the media and government for obvious reasons.

They make a lot of money, but not enough to justify a re-considering of the legal issues involving them, and hence, a re-thinking of the moral issues against them (e.g., young girls aren't supposed to attractive to adults, dammit! Adults are supposed to consider the sight of a 12-year-old girl in a bikini to be disgusting!). There is also the additional emotional issue of the administrators of these cyber-locales trampling on sacred ground by creating such sites, that ground being the sacrosanct paradigm of kids personifying "innocence" to Western culture. The fact that teen girls under 18 are legally considered children results in they're being pulled into the paradigm and forced to adhere to it too.

The administrators of these sites have often been parent, the kids themselves, or professional photographers, but that is only going to result in: 1. When parents are the administration, they are demonized as bad parents despite the frequent arguments that parents always know best and should make the decisions regarding their kids instead of the state; 2. When kids are the administrators of their own sites, we are subjected to the "where are the parents!?" rants and the kids are told that they never know what's best for themselves (unless they tell the public what it wants to hear); 3. If professional photographers are the administrators, then they are accused of being predatory exploiters, blah blah blah.

And, as you said, if there is a money trail, it becomes too easy to track down legal sites of that sort and subject the administration behind them to massive public shaming until they ultimately conclude it's just not worth it anymore and shut down. This one-by-one picking off of these sites occurs before enough of them can get up and form the equivalent of a united front or union as protection from media and government assaults.

Nowadays, with the underage model sites more or less gone, MAPs have had to rely on what is available in the public sphere on YouTube, TikTok, Instagram, et al. to admire beautiful young girls being their amazing selves. The problem is, as expected, the public is aghast over this, numerous YouTube channel owners make it a point to virtue signal by devoting entire videos to attack these channels, the people who run them (regardless of age); and the people who watch them. The resulting combo is mass public shaming, shadow banning, shut downs, and investigations make it very difficult for channel owners to keep such videos up.

Of course, there is also always going to be one or two girls that complain they were genuinely exploited, and even if true, they will never be treated as outliers no matter what the rest of the girls said -- down the site goes. And it can be very difficult to tell these days if the girl is telling the truth or if she was a "plant" by external influences to make the right accusations that only a single girl has to make to get the site taken down and owners arrested. In fact, I am almost certain the latter was the case regarding the Webe Web network of child modeling sites based on the strange circumstances of how it went down and the set of accusations that would likely not have held up if an adult woman made them.

When the above happens, we are stuck between the rock and a hard place of genuinely wanting to make sure girls working for such sites are being treated well and the understanding that it's very easy to take these sites down wing a single young individual being hired to "cry wolf". Many potential site administrators realize this and do not think the potential pitfalls of running such sites, no matter how well the girls are treated, would be worth it in the end. Hence, the industry inevitably died out.

Finally, the way the public sees it, MAPs are so disgusting that we do not deserve any way to satisfy our desires via fantasizing at all, and that any external opportunity for us to do so in the quiet confines of our home in a safe and legal manner that in no way involves real minors should be identified and stomped out. Most people across social media also try to make themselves look as virtuous as Jesus Christ himself by loudly proclaiming their opposition to any video, film, etc. that in any way acknowledges the attractiveness of youth or the fact that kids, too, are sexual beings who derive pleasure from advertising their attractiveness to the public in the hope of acquiring a legion of admirers.