Sorry to bring this up in an old thread, but as a movie buff who's seen more than a few horror films, here are a couple observations.
1. One reason is that many vampire films portray vampirism as being more of a scientific/ medical condition than a supernatural one (i.e. the "Blade" series, in which there is a medical Latin name for vampires).
2. Our society, especially the media, is becoming more secularized.
3. Some vampire films of today do have religious weapons used. For instance, the "Lost Boys" series has the vampire hunter Edgar Frog (Corey Feldman) relying largely on religious weaponry. SPOILERS AHEAD! In the lame second movie, "Lost Boys: The Tribe," Edgar at one point reveals that he got himself ordained over the Internet so he could have the authority to bless water and turn it into holy water, and even has a pretty cool makeshift grenade launcher that he uses to shoot holy water balloons at vampires. In the climax of the MUCH better third film, "Lost Boys: The Thirst" (I recommend at least renting it, it's of course not a classic like the 1987 original, but still a quite enjoyable movie), Edgar destroys the main vampire by busting open a small regular water pipe so it'll spray onto the main vamp, then immediately blessing the water so that it turns holy and annihilates the vamp. SPOILERS DONE.