"Oh, some of the crowd is interested in the Hellmouth or drawn by the size of the existing crowd. But plenty of demons and vampires and assorted magic users have no idea the Hellmouth is even here, and yet they still show up, in numbers vastly disproportionate to the size of the town. I'm not enough of a magical theorist to know if the amount of miscellaneous magic that clusters here without the obvious intervention of any intelligent beings is adequately explained by the presence of a great big gate to hell, but there's a lot of that too."