Fun question! Let's see...
I guess I write YA, because I like to read YA.
The main three things I like about YA are the length, the protagonists' ages, and the fact that so many of those protagonists are female.
I read mostly fantasy but so much of adult fantasy is sooooo long. I used to love getting sucked into 1000 + page epic fantasy novels, but now that I'm a working mom, I don't have the time or attention span for those long epics. YA still gives me good fantasy stories at a more digestible length.
I also enjoy reading about young people because it's such an exciting part of your life and one I miss. (I'm getting kind of old
)
And, of course, I'm a woman, and I enjoy reading books from female perspectives. I'm not against a male protagonist per-se, but I got tired of the lack of strong female leads in adult fantasy books. Once I clued into the fact that YA fantasy was riddled with female protagonists, I was hooked.