The reason given in the books (according to my wife) is that it was basically "evolutionary" - vampires are that way to make them more attractive to humans, sort of like how predatory plants are attractive to flies and other insects.

Werewolves were also an "evolution" to protect the Quilute tribe from extinction at the hands of vampires.

Of all of Meyer's decisions that I'm aware of, I thought these were by far the most clever.