For the bazillionth time, what is this thing we call YA?

So there I was, reading Ysabel, (which is a fantasy, but also about history and family and coming of age and some other things*) when I had the thought that probably everyone else who has read the book has had: 

So.  Why wasn't this marketed as a YA novel?

Obviously, there's the... obvious, which is that the author is already a known quantity in the grown-up world.

But then there's also this:  Even though the main character is a fifteen-year-old boy, and even though his aforementioned coming-of-age is a big part of the story, Ysabel didn't really feel like a YA novel.  To me, anyway.  Maybe because YA novels tend to have more of a feeling of intimacy and immediacy and this one made me feel like I was witnessing the events from a bit of a distance?  I don't know.  Thoughts?

*Which I really enjoyed, by the way, and which I'll blather on at greater length later, probably for O' Canada Day.