The Hellmouth is an area :
in which the barriers between dimensions was particularly weak, allowing the creation of portals between Earth and several hell dimensions. For these reasons, the Hellmouth attracted demons and other supernatural creatures, becoming a "hot spot" for supernatural activity.
The most famous one was located in Sunnydale, right below the high school attended by Buffy Summers. Another was humorously mentioned to be located in Cleveland. Aside from those two, no others are known to exist.
So, why do Slayers bother living anywhere else?
It seems that Buffy and Faith are the only Slayers to ever take up residence in the area. The other Slayers shown on the show are located in Jamaica, China and New York City. The show never mentions any other Slayers as having history with Sunnydale or Cleveland. If there's a limited number of points on Earth where all of the demons are going or coming from, wouldn't it make sense to live there? And seeing as how most of the Big Bads of Buffy need the Hellmouth for their world-ending plans, wouldn't it make sense to guard it? Did evil-doers just not use the Hellmouth for apocalypse-things before Buffy arrived?
Is there a canonical answer, even one given in novels or comics? Or is there any rational explanation?
And I would imagine that "they didn't know it existed" explains it. It seems like the Hellmouth's whereabouts is as old a knowledge as any other, and definitely written about in the dusty old books as anything else on the show. Especially as modern technology and travel/communications came about in the 20th century, knowledge of the Hellmouth's location would be made available to the active Slayer.