Why do people in the audience hide their faces?
you can see them wearing mask and trying to hide even their eyes
(is it really just because they are shy? )
It's a cultural tradition with long history behind it but simply put it's about not showing one's teeth, specially a maiden's, as they are not only considered to be beautiful but also polite and respectful
What you have to realise tho is that they don't do it consciously and go "i have to protect this tradition to honor my ancestors" , it's simply a reflex to them- as natural as breathing or walking; they don't ask why, they simply do.