For example, there might be 400 people wanting to fly ABC to XYZ every weekday, but some want a morning flight, some want an evening flight. Operating a 747 on this route would be a bust, it would be only half full. A 757 or comparable flying twice a day would make good money.
One of the odd things I noticed about travel in Asia is that the likes of Cathay, SIA and the Japanese can run 777s and 744s on short hops and still make a decent profit, and yet for some reason it's almost impossible to do this anywhere else in the world.