This has been covered here before, but the issue with winglets is that they are very effective, but the aircraft has to spend most of its time in a particular flight regime as they are idealized for just one phase of flight. The winglets on the MD-11 actually increase fuel burn during the climb and descent phases, for example. The airlines wanted the 777 to be efficient for both short and long haul, hence the wingspan was increased and they did everything they could short of winglets on it. A purely long range 777 would benefit from winglets, so far there aren't any!