I kind of see YA, fantasy and sci-fi as three different sets of common elements. So, there is an intersection between YA and fantasy just as there is a (relatively smaller) set of YA and sci-fi or even YA and urban fiction. Taken apart, there are definitely some distinct things that make up a YA genre to me, namely how they deal with certain issues.

Beyond that, I don't think it is really a gap. Just the intersection of YA and fantasy is actually a rather large intersection compared to the interaction of "adult fiction" and YA fiction. There is more in common, so there are more readers who read both. :)

Set theory is fun.