Why is "Lucy" considered it's own species, when they only found one?

Just because Lucy was the only member of her species discovered doesn't mean that others did not exist. Also, humans did not evolve from chimps. Humans and chimps evolved from a common ancestor. Some of this common ancestor evolved into the monkeys we have around today, and some evolved into humans. The reason that other species have died out is complicated. Natural selection plays a big role. You should really consider reading "The Origin of the Species" if you want to understand better how evolution works.