I have a few possible reasons. One, most fics are written by females, and it's easier for females to write female characters. Two, boys are harder to write than girls because with girls you can do the whole emotional, "why didn't you call me?" or "I missed you" thing and I just don't see boys saying that to their mother, and crying.

And also, I think this secret daughter thing occurs in books and movies etc more than the secret son thing...