Quite simple really. Doctors are educated people who are taught to believe that they are the brightest, sharpest and never wrong, or at least shouldn't be seen to be.
If they can't diagnose an illness using their standard tests and can't offer the patient any help, this makes them feel bad and a little inadequate. Doctors don't like this, they don't like or respect patients opinions and in fact many doctors don't even like their patients. So if they can't be wrong and their tests can't be wrong then the patient must be wrong, ergo a psychiatric problem.
Luckily for them they have a symbiotic relatonship with their psychiatric breathren. Classifying a patient as having a psychiatric illness relieves the doctor of having to deal with a patient who frankly makes them feel bad while delivering and funding patients to a branch of medicine that really should have died out last century.
Doctors, psychiatrists and politicians are all happy. The problem patients are gone, the treatment regime is cheaper and it plays to their pre-conceived prejudices of the deserving and undeserving poor.
With everyone happy (almost) its easy to dismiss any physical evidence as fringe, not-replicated, propaganda, abberrant or, given that the condition is now regarded by the consensus and psychiatric, ideopathic and therefore not worthy of consideration.
Of course once the truth comes out, as in H Pylori and stomach ulcers, its all absorbed into the mainstream. Quietly, without fuss, apology or compensation.