Most Helpful Girl
Because Joss Whedon willed it to be so.
Probably because her prime directive was to kill vampires; she just happened to move to a Hell Mouth, where there were a crap ton of other monsters, and had to abilities to stop them. I don't think she fought, or even knew about other monsters until to moved to Sunnydale. Also, I think there were only vampires in the original movie, and the kept the name and title from that.