Well here are a couple of reasons:
1. Vampire Hunter D's story is strikingly similar to the Castlevania Alucard/Dracula story, almost as if they belong together, even if they are completely different universes.
2. the obvious similarities between characters, such as the aforementioned D/Doris and Al/Sonia.
3. Castlevania and Vampire Hunter D came out almost around the same time. They are sibling series.
4. This thread makes mention that Ayami Kojima, a designer for Castlevania, is now designing for Vampire Hunter D, now making the universes even closer together, now artistically.
5. A person connected to the Castlevania franchise is now doing other works.... sort of a "Where are they now?" of Castlevania important people. Thus, it's still Castlevania-related. Even more so if the works mentioned are already similar.