Because I'm too young to write anything else.
No! That's only part of it. The real reason is this:
Endless possibilities, for me and for my characters. They can fall in love for the first time, travel the world, learn who they are, discover what they want in life, take down the bad guy, or they can do it all. So can I. I can be what my generation strives for. A little behavior that I like to call genre-bending.
I am a genre-bender, and YA is the ultimate platform for it. Adventure-paranormal-scifi or historical-thriller-romance or western-fantasy-horror. So on and so forth! Wait wait wait! Did I just say western-fantasy-horror? Hmm...maybe I should try to pull that off someday...
In the adult world, things get all separated. Books can be literary, or they can be "commercial" (a most problematic label). Books need to find a home in the bookstore, meaning that writers have to fit their stories into those categories. They can draw on multiple elements but ultimately their book has to be suspense OR romance. Horror OR fantasy.
As an aspiring YA author, I don't have to do that. I can have it all. That's not to say there aren't limits. My main characters must be approximately between the ages of 14 and 19, and must deal with coming-of-age issues along the way. But I like that. They have all the possibilities in the world open to them. They are just beginning their crime-fighting or bad-boy-loving or zombie-slaying lives.
I feel their freedom.