Because sometimes a stake and some holy water just isn't enough to take down the vampire hordes.
No... what you need is a good solid steampunk tank to mow down the ranks of the LEGO undead.
Castor Troy has actually designed an entire range of steampunk styled vampire hunting vehicles. If I was Dracula, I'd be a little bit scared right now.
On the other hand, having seen photos of the LEGO Vampire Castle that Castor Troy is building (it's a WIP going on a year so far) this machinery still may not be enough.
Source: Vampire Hunter: The Tank