A. That's what I write.
B. They're usually short enough so I don't have to put them down in the middle of an exciting plot.
C. They aren't depressing or sordid, and
D. I'm still a kid at heart.
To tell the truth, the first reason is really just an excuse. I'd read that kind of book even if I didn't write them. Or maybe I'm confusing cause and effect; I write books for kids because I read them.