This might have been said already, but I'm just gonna give my two cents.
The Masters rising coincided with Buffy's arrival, so there would have been 'more' mystical activity once that occurred since the vampires would be attracted to the masters presence? also, let's be realistic here, in a sense, the place attracts demons and such but that can't mean that the whole of Sunnydale would be overrun with them, and for the show's sake, demons would only come here and then, others might be in Cleveland for instance.
The Masters rising coincided with Buffy's arrival, so there would have been 'more' mystical activity once that occurred since the vampires would be attracted to the masters presence? also, let's be realistic here, in a sense, the place attracts demons and such but that can't mean that the whole of Sunnydale would be overrun with them, and for the show's sake, demons would only come here and then, others might be in Cleveland for instance.