Biomedical specialization is a relatively new phenomenon, with its roots in the inclusion of surgery into medicine during the 19th century. When the formerly separate fields of surgery and medicine combined, so too did the range of procedures, skill sets, and knowledges needed to support a diversifying medical profession. Later, as scientific knowledge about the body expanded and medicine aligned itself with the sciences, there was greater specialization typically divided by organ systems.

Dermatology is an interesting case because exterior skin conditions are visible, and could be treated and examined before studies in anatomy, microbiology, and physiology permitted physicians a look at the inner human body. Poultices and ointments for skin conditions have existed in some form across the entire history of medicine. As a physician specialty, dermatology emerged in the early to mid 1800s around the same time as other medical specialties, as there was a growing division of medical practice by organs as I described above.

As to why dermatology arose as a medical specialty: this was in part due to the growth of scientific knowledge about the body that made it more and more impossible for individual physicians to have working knowledge of all systems and diseases, so they began to pick and choose (i.e. specialize.) It was also in part due to the business interests of the medical profession. Especially in Europe and the United States, physicians wanted to consolidate their control over all other forms of medical practice-- not only because they had access to the kind of scientific knowledge needed to understand complex ailments that homeopaths and other, traditional medical laborers did, but also because they could replace more homegrown brands of practitioners (i.e. the local guy selling the tincture he brewed in his kitchen down the street) and take their business.