To oversimplify things, Derm has it all. Comparatively healthy and appreciative patient population, tons of interesting pathology (in academic centers, at least), a mixture of procedures and medical dermatology, light call, reasonable business hours, and reasonably good pay.
Your question seems to be asking "why would someone want to do Derm when they could work much longer hours for the same amount of money?" I'm having a hard time figuring out why someone would be confused about this.