Because purple fits more naturally into the skin tone. Rather than being awkward like green or blue and greying it out. Choose the right purple can make skin pop while being darker at the same time.
I know, I was just bringing to attention the blue in the skin tone. Cos SEEEE it's okay to use odd colors, we get weird colors on our bodies in real life, why not use them for the art we make? =3
Aw, just tell her to try it out on a doodle she doesn't care about, and when she sees it doesn't ruin things maybe she'll be more willing to try things out?