Just supposition but a few reasons I can think of…

There must be a lot of crossover between people who have the skills to host an instance, people who have the time to host an instance, and people who enjoy this type of imagery.

Also, there’s no honor among thieves.

These guys persons are flirting with danger in many cases. I’m pretty sure the law in my jurisdiction considers anime to be CSAM if a reasonable person is likely to conclude that the person depicted is a minor.

What I’m saying is someone might be willing to walk that line with whatever niche community they want to host because they can manage their own risks. However it would be a huge risk to host other communities lest you fall afoul of the law.

A lot of these are adjacent to real observable phenomenon but a nutty belief system has been overlaid and then additional claims are made on the basis of that nutty belief system which are not observable.

For example, Feng Shui in practice is usually pretty sensible “where should I put the sofa” kind of stuff, but if you claim that it’s about the flow of qi through your house and suggest that based on that not only should the sofa go over there, but you need to put a topiary vase on the table next to it, that might be a nice aesthetic touch but there’s no evidence of qi.

Additionally there’s plenty of Traditional Chinese Medicine that became actual medicine because it has observable properties. For example turmeric is a mild anti-inflammatory.