9 of Cups as the wish card is traditional. I don't know what tradition or where it came from, but traditional!
I see the 9s a karma cards--you get what you deserve.
So, since the cups are emotional and usually positive, I think that fits right in with: You deserve and will get what you wish for. Of course, when I see it in a reading, I always think, "be careful what you wish for because you might just get it!"