I don't think Castlevania's marketing problems are to do with vampires and stuff. It's more about un-aggressive advertising campaigns and boneheaded moves, almost as if Konami doesn't really care. Take Adventure ReBirth, for example. If memory serves, it was announced roughly about two weeks before its release, and it was in some kind of offhand manner (I think it was a small blurb in a magazine, or something). Then, no one really knew the release date. Even when the game was out, plenty of people (even hardcore Castlevania fans) weren't really sure it was even out. There was confusion galore. No promotional art, no ads in magazines, no hype... nothing. It was just there. Quietly.
But yeah, for the most part, Castlevania is played by, for lack of a better term, ordinary people who like video games. Not primarily vampirefreaks.com people who are a part of a very distinctive "scene" or indulge in some weird alternative lifestyle. I hold that people who like great games won't be turned off by vampires, decrepit castles, and other horror monsters.
Aside from marketing issues, the problem with Castlevania is that there hasn't been anything earth shattering and status quo breaking while at the same time being GOOD since Symphony of the Night. While good games have been released, there hasn't been anything that makes a huge statement and puts the series in a new and fresh direction that has been generally pleasing. Lords tried to do that, but it failed. Because of this stagnation, some people don't feel the need to pay attention to Castlevania. Now, things are worse than ever before, because the games aren't even good.