Santa, being busy at the time said "I don't know, I'll worry about that later."
But the little angel kept bothering him. "Santa! Where does this Christmas tree go? I need to know! Where do you want me to put the Christmas tree, Santa!?"
Santa, in a fit of rage finally yelled back. "You wanna know where you can put the Christmas tree? I'll tell you exactly where you can put the god damn Christmas tree..."
...And that is why the angel sits on top of the Christmas tree to this day.