High fantasy, huh?
Some very, very clever and determined (and OLD) Vampires have collaborated and found a way to put this particular city into eternal night. Nobody knows how they did it, the wizards are frantically trying to undo it, but the Vampires are very sneaky and have managed to hide, thus avoiding answers.
That makes the townsfolk remaining extremely wary. And paranoid. Sales of vampire-Be-Gone are through the roof.
That way, you neatly sidestep having to explain anything, providing the reader with a good hook to suspend his/her disbelief, right off the bat.
No pun intended. (Bat, vampire, get it? Huh? Huh?)