The impression I've gotten from the reading I've done on these sorts of conditions is that part of the reason doctors are reluctant to do research is because it such a fraught area that anyone who engages with it are targeted by groups of sufferers. Even researchers who favour a biological cause can be subject to unwelcome attention and harassment.

These sufferers are not all, and probably not even most, of the total, but they are organised and militant, and fully convinced of two things: That their condition has a 100% biological cause, and that they cannot trust the medical and scientific establishment. Even if they are completely right, it has created a situation where the cost of getting involved as a medical professional are just too high, so it has become a self-perpetuating situation.