Several reasons: first, Dramatica deals with a structural view of story. Most story terminology deals with the techniques for telling a story, not for describing its structure. Dramatica looks at ideas that are fundamentally different than what most writers have been taught.
Second, there are no fixed definitions for many of the writing terms writers take for granted. One experiment that we tried was to ask 100 writing teachers questions about basic concepts of story. Questions like, "What is the concept of theme mean?" and "How do you know what happens in the second act?" What we got was a wide variety of answers, with no real agreement on what the most basic story concepts mean. So while writers and teachers of writing all have an internal understanding of basic writing concepts, the actual definitions for those concepts vary widely.
Only a portion of Dramatica's terminology is "invented" ("contagonist", for example). Invented terms generally cover concepts that are unique to Dramatica. Most of the other terms are familiar and mean exactly what you think they do.