Because roughly 50% of children are girls, while less than 50% of programmers are. I just backed this to give to my son and my daughter.
The book is aimed at girls. It would be nice to think that children were not conditioned by gender stereotypes at the age of 6/7, but unfortunately they are.
I'm going to believe the author when she says that this is for children. I just don't buy the idea that boys can't enjoy stories with female leads. Look at Brave or The Hunger Games. My son loves those.