Because their entire perception of derm is based on a private practice cosmetic derm in NYC/LA/Beverly Hills or based on watching a Proactive commercial. They don't know about the ridiculous amounts of reading you have to do once you go home, preparation for conferences, or grand rounds. Not to mention, at a real academic medical center, you will see much more than just the mundane skin afflictions of suburbia. There are some IM attendings who realize how interesting Derm can be and can see beyond lifestyle.
Especially in the House of Medicine, if you're not stenting a vessel, sticking a scope up someone, pumping someone with IV fluids, or doing a major abdominal operation, etc. then you're not doing real medicine, as far as they're concerned. Internal Medicine (I'm assuming that's your prelim) is one of the worst when it comes to believing it's better than everyone else, bc at one time, it was the specialty that those at the top of the medical school class pursued in droves.
Not too long ago, it was well known that those at the top of the medical school class tended to go for Internal Medicine and General Surgery. Believe it or not, Ortho was for people at the bottom of the class. Same for Radiology and Rad Oncology (known as Therapeutic Radiology) - which were filled in droves by IMGs bc at that time there were only X-rays as MRI and CT hadn't been invented yet.