In the first seasons of Buffy it seems that demons and vampires are not very common. One Slayer seems enough to keep things under control (particularly in the hellmouth).
By season 6, it seems that there are demons absolutely everywhere and they are pretty mundane (there are demon bars!). They are as common as bike gangs (as seen in season 6 premiere). It seems that the only reason Sunnydale has not been completely terrorized is the presence of the Slayer.
So, why are not the rest of the towns (particularly ones close to Sunnydale) in that situation? One or two Slayers cannot possibly maintain order with the amount of demons that seem to populate the earth.