The reason is that the 777-300 dosent have the range as the 747-200 has. It was indented onlt for smaller routes around South East Asia. The new 777-300X with engines in the 100,000 lb. category with a fuel tank in the aft cargo. Together with reinforced tail surfaces and futher-strengthened wings with extended tips that probable not only have equal range but increased as well. This is a true replace ment for the 747 "Classics" on the Boeing side.
On Airbus thats all ready for the ordering books, is their A340-600. Which competes the 777-300X headone for the replacement of the 747 "Classics". The A340-600 is expected to have a range of 7,400 NM. With an all new engine with 60,000 puonds of thrust. I think they are Rolls-Royce ones.