Well, it could be that CX had already been operating the A340 for some time before the first B777 was built. And since they already have A340's and B747-400's, they don't need to add yet another long-haul type to their fleet. Obviously, many factors go into an airline's fleet decisions. Why did Air France choose the B777-200LR over the A340-500? And why has Singapore decided to operate both the B777-200ER and the A340-500?