Because the manufacturer's logo is usually in the lower half of the face. Placing the hands at ten to ten looks neat (hands not too close together or too far apart) and doesn't obscure the logo.
Try to think like an advertising psychologist - subliminal messages are hidden everywhere
hands at 10:10 (or 14:50) = happy face
hands at 08:20 (or 16:40) = sad face
Would you rather buy a watch that looked happy or sad?
Just kidding - and fanning the flames of the conspiracy theorists.