Despite their small number, there are Young Adult books which have minorities or their starring characters are minorities. But their stories often focus on that which makes them a minority. For example, most LGBTQ (lesbian, gay, bisexual, transgender and queer) literature falls into the romance category. Books featuring disabled characters will focus on the characters overcoming their disability.
These books are great -- but minorities are rarely ever normalised in Young Adult literature. There is also a lack of disabled characters in young adult sci-fi books and a disproportionate racial demographic in dystopian novels.
A greater range of diversity within the main characters of Young Adult literature would create a larger demographic of readers. Young Adult literature has the potential to become an educational resource for young people about cultures outside of our western society, and different sexual identities -- provided authors provide a more diverse cast.
Most importantly, everyone enjoys reading about characters that they relate to. Many people are unable to relate to the characters which currently populate Young Adult literature. Finding characters to relate to brings comfort to the reader and the main character of a book is the person that the reader is most likely to become attached to.
This means that it is especially important to have a racially, culturally and socially diverse range of main characters. We're not seeing that in our favourite books.