Buffy already had enough on her mind with the Hellmouth under the school library opening up.  Sunnydale was home only for a measly few days before it decided to spew the horrors upon her and her newfound friends.

Sure, she was "The Chosen One" but there was another with powers like her own on the other side of the country.  What the watchers didn't perceive until it was too late that another portal had opened up, a "vent" for the Hellmouth.  When Buffy died -- the first time -- the watchers just assumed that Buffy's powers were given to Kendra.  In actuality, the appearance of the "vent" coincided with her death, and caused a second slayer to have the power.

Now demons and other nasties have found another way into our world.  They're not quite in such an abundance as they are in Sunnydale but they're still evil and smell funny too.  That would also explain to the watchers why there could be two Slayers in the world, and why Buffy always has that one extra edge over the others.  It was actually Faith and Kendra's destiny to watch over the vent but the strength of the Hellmouth drew them there, instead. 

With them out of the way, the second slayer needed to be found and keep the baddies from taking over.  In the attempt to purge these new nasties from this world, the Council sent in a new watcher to the states to find the new slayer.