There is a long history to this. Psychiatry dates from about 1890. As a science it is restricted by ethical considerations from performing the kinds of experiments biologists can do with lab rats. It took decades of close observation, comparison of case studies in international conferences, and much theorizing to categorize mental disorders. Freud's theories concentrated on childhood causes of anxiety, especially sexual repression, and couldn't account at all for psychotic states. The shell shock of WWI and WWII baffled psychologists. When people did come in for treatment, the model at the time was to listen without comment as the patient expostulated freely. This talk therapy took years to provide insights, and even after insights, change was still slow.
In the 1970's psychiatry began to abandon this model. The new emphasis was upon clinical observation, and practical help at the expense of theories. That's what enabled the science to finally identify PTSD as a true anxiety disorder. And the new emphasis upon observation revealed a host of other previously undiganosed disorders as well, such as attention deficit disorder (previously such kids were just brats).