Because one thing I like about YA is its flexibility. Since there are no genre boundaries, readers have more opportunities to become exposed to different varieties of fiction. And writers have more opportunities to cross genre lines.
But that's not the only reason YA is more flexible. Perhaps a bigger element is the target audience. With more readers growing into YA (and some growing out of it) the core audience is constantly shifting--which means that YA is much more easily able to adapt to change. (The fact that it's a newer category helps, too.)
I went to a few critique meetings with local SF/F writers. Most of them are in their forties (at least) and it's weird to think that many of them have been reading SF/F since before I was born. Earlier today (read: a few minutes ago) I was talking to someone on Twitter about how women have difficulty breaking into SF/F genre and culture (outside of urban fantasy). And a big reason for this is the fact that the same people have been reading, writing, and publishing in the genre for such a long time. That, and the fact that it's somewhat of a niche audience, which means it's very difficult to convince fantasy publishers to take risks.
This is where YA comes in. Right now, YA is the most profitable genre on the market. This is a good thing. The books that become successful in YA are the ones that will end up crossing over into adult shelves. And when adult SF/F publishers see more female-centered books becoming successful, they will be much more open to allowing more of them to exist.
BUT, this change will be slow. Publishing is a slow business, and it will be even slower if all female writers abandon adult genre fiction in favor of more gender-balanced YA. So how do we speed up the process of giving women an equal presence in all market?
Simple. Buy more female-friendly books IN THOSE MARKETS.
Want recommendations? When I first started reading adult SF/F, I didn't know which authors or books were considered the most popular--I just picked up books that sounded good. As a result, I discovered Green Rider and A Sorceror's Treason. It's been about ten years since I read these books, and I can't say what I would think of them if I read them today, but as a teenager, I absolutely loved them.
And what about the Otherland quartet? This is one of my favorite series. One of the many reasons for this is because of Renie, the protagonist. She's probably one of the strongest female characters I've ever seen, and that's without having kickass fight skills or magic powers.
And finally, I should probably mention Who Fears Death. I'm not sure I would consider it entirely feminist, as the love interest is controlling and falls well into asshole territory, but there are other elements that I love about it--like how it acknowledges that women enjoy sex and that it's possible for a girl to sleep around and still be loyal and heroic.