A few starting notes:
Because she has a rocking story arc! Because the change in Linda is... I think Dee Dee says it better:
Sarah is strong - she struggles a lot with what she's going through: both the attitudes of the white students/teachers, and the fact that she is attracted to girls.
Recent discussion of this book has brought up issues with the racial representation in this book - especially the use of the 'n' word, and issues with Linda's early perspective.
Here's a review from Sammie @ Bookshelves and Biros, discussing problems with the racial representation in this book.