I was once told in a writing group that it was time to move on from high school and start writing grown up fiction. I smiled politely and said nothing, though I had a few choice words in mind. Write for adults? Where's the fun in that? YA stories are full of all the things that make adult book's good, such as a great plot and interesting characters. The difference is, YA stories are usually stuffed full of energy and shoved into a much smaller package. They are generally 50,000-70,000 words as opposed to the 70,000-150,000 words of adult fiction (or more if you are certain fantasy writers).
Of course, it's also just a personal preference. There are a lot of writers who want and need to go into a lot of details about the history of the world where the story is based, or explanations of what exactly that tree looks like. That's what their story is about, the details. Or perhaps they want the extra word count to create an extremely complicated plot with numerous characters. For them that works. I'm not one of those people. I like stories that get to the point and fast.
YA also has such a fun voice. Teenagers are complicated creatures who are going through a huge turning point in their life. They practically build their own language to deal with everything they are going through. Although I would have no interest in becoming a teenager again, I definitely think it's an exciting time, for good or bad. And that leads to some great fiction.
So why do I write young adult stories? Because that's what I read. That's what I love. And that's what comes out when I write.