same reason we tilt our head to the side slightly when we are listening to someone and want to appear interested or introspective. It's called the Divine Tilt in art history. Jesus and Mary also always had their head tilted in paintings, it's seen in Pagan art work too, i.e. The Birth of Venus.
Basically, it conveyed an image of piety. Filtered down into body language and social norms, generation to generation, until today, when you'll see that Hillary Clinton tilts her head whenever listening to a question, or your 16 year-old cousin tilt her head in every Facebook photograph.