Why are downward pointing winglets more efficient?
For efficiency the orientation of winglets is not important. Best would be to stretch them out horizontally, but on a canard they do double duty as vertical tails.

Things look different when maneuvering is added: Now the downward-pointing winglet is superior especially if it carries a rudder. When roll is commanded by deflecting the ailerons, the lift change on the outer wing will carry over to the winglet. On upward-facing winglets this creates an opposite yawing moment while on downward-facing winglets this yawing moment supports the intended turning maneuver.

Also, they look cooler.