Because she's a title character. They shouldn't have to stick stories focused on her in the back of her own book. Imagine if they pulled that with Superman/Batman: Superman runs around saving lives and giving big heroic speeches while Bats spends the entire book being kidnapped, ignored, or inept. That, and the fact that they're going to make Dinah awesome in the back of the book probably won't make them any more inclined to make her awesome in the front of the book.