@ Anita:

Because it was one of the few performances where she didn't strain her voice beyond its range. Like Remmy said, she wasn't competing with loud instruments because it was acoustic. There some live performances of her before she got famous where she wasn't performing with a band, it was just her and her guitar, and she sounded decent. Like this one here: http://www.youtube.com/watch?v=v4D1kNH6O1o