Did you list all your prelims independently on your MAIN rank order list below all your derm ranks), and not just on your supplemental ROLs? This is necessary to match into just a prelim if you don't get the derm spot.
6 interviews for derm is pretty low. At least 12-14 seems like a solid number to have a good chance of matching. With your credentials, you should be getting that many interviews if you applied broadly.
Also, a lot of IM, TY, or Surgery programs with prelim spots already have a bad vibe for prelims. They see them as generally being smarter and having better credentials on paper than their categorical residents, but the prelims are not interested in IM or Surgery and just want to "make it through the year" with the minimum amount of work (ie want a cushy prelim year). If during interview day you start asked questions to anyone such as the number of weeks of vacation you'll get, how much wards/ICU versus elective time you will get, salary/benefits, overnight call, work hours, or the patient volume you're expected to handle, this would have raised a major red flag to programs.