From my understanding there aren't alot of vampires in the world...the folks in Mystic Falls thought they had the problem solved until recently.
If you're a normal human who knows about vampires and wants to hunt them down, why not just tell everyone they exist?
Your asking a question as a fan though...I am sure LJ Smith with the books and the writers with the show don't think it would serve their story by everyone knowing. The same could be said with other series that have the public in the dark while there is a select few who know and deal with the problem.
 Plus the public knowing and dealing with vampires seems like a different type of story than what they want to tell...maybe you should write that...a story about humans and vampires living and working together.