Because YA deal with emotional growth and learned experiences. Not to mention it’s faster paced and is less of an overall bore.
Some people think YA books are dumbed down, but they’re not. They’re stronger in a lot of ways. Anything goes in a YA novel, the only thing that is a must is that the main character must relate to other teenagers. But any scene will be stronger in a teenager than an adult because of their reactions to it.
In Layman’s terms: it’s the deep emotional connection between reader and character that keeps me reading and writing the YA genre.