From my understanding, the leading theory of what causes autoimmune diseases is viruses. The immune system responds to the virus and ends up mistakenly attacking the body as well. This seems to support that, and shows that different viruses cause different autoimmune diseases.
I’ve been reading recent research on MS and other autoimmune conditions. I also study APS1 in my PhD at the moment (caused by AIRE gene deficiency). I’m definitely not an expert, but I don’t think it’s a leading theory. As always in biology, autoimmune diseases probably have dozens different causes. Many of them might be environmental or genetic. Better yet a combination of the two.
Broadly speaking, the hygiene hypothesis is the current leading explanation.
To make an analogy, in AI/ML terms, it amounts to a training set problem: you're not exposed to a diverse enough set of microbes and pathogens, or you're exposed to a biased sample, or you're not exposed to enough microbes.
Your immune system's role is to act as a classifier to discriminate between good and bad, self and non-self. When you have a bad training set, you increase the odds that good things will be identified as bad, or that self antigen will be recognized as foreign.
Parasites. When you start getting rid of parasites in 3rd world communities, auto immune issues start to crop up. It's thought that since the body isn't fighting off parasites then it sits idle and starts attacking itself. It kind of makes sense from a purely evolutionary perspective. There's actually been treatments for some auto immune diseases that involve giving a relatively harmless parasite to people.
If your country doesn't have a medical system that will detect such things in the general population and does not have a system to get the numbers for statistics, how are you going to know how prevalent something is?
We don't know how common lots of things are in 1st world countries either, simply because we don't track.
There's also some really compelling evidence that psoriasis is caused by bacteria. Psoriasis is often described as autoimmune, but is probably better described as immune-mediated disorder. Unlike autoimmune disorders like MS and lupus, psoriasis doesn't involve the body "attacking" itself.
Specifically, streptococcus bacteria such as Streptoccocus pyogenes, also called Group A Streptococcus, which is primarily responsible for rheumatic fever and scarlet fever in developing countries, and more commonly known in the West for causing strep throat (streptococcal pharyngitis).
We already know with almost certainty that S. pyogenes triggers a form of psoriasis called guttate, which in 60% of cases resolves on its own within 1-3 months. But in the remaining cases, it turns chronic. It can also come and go: spontaneously resolving and then coming back a few months after.
What's particular about strep is that it appears that it can lie dormant in the body. The tonsils of psoriasis sufferers have been found to contain strep bacteria, and a recent meta-study concluded that a tonsillectomy can significantly improve psoriasis in about 70% of patients who undergo the procedure; so it's likely that the tonsils act as a reservoir for continuous reactivation. (Apparently, people who have a tonsillectomy as a child are also less likely to develop psoriasis.) Strep bacteria are also thought to evade serological detection by hiding in biofilm, from which they periodically emerge to reactivate the immune response.
As to why only some people develop psoriasis, the explanation might be some kind of genetic predisposition. Psoriasis is often explained as maybe being caused by "cross talk" between the adaptive and innate immune systems.
Some papers:
Streptococcus pyogenes-induced cutaneous lymphocyte antigen-positive T cell-dependent epidermal cell activation triggers TH17 responses in patients with guttate psoriasis
https://pubmed.ncbi.nlm.nih.gov/27056267/
More info: https://www.nature.com/articles/d41586-021-01835-w