Great question. Easy answer. Most Dermatologists are conservative and are too busy to get updated in every single field of Dermatology. They usually read or hear lectures about the specific specialty they are interested in. For many years, many of us were skeptic about the relationship of diet to Acne. Nevertheless, an impressive number of recent studies have proven a strong relationship between Diet and Acne - especially for Cow Milk and dairy.
Here is more info on acne or cystic acne and diet, that will provide a good guideline to get started on cleaning up your diet and acne.