There are some airlines, however, that make economic use of the 773 - CX and SQ are 2 in particular. Because they have lower weights, because floor space is the same, if range is not an issue (e.g. 8 hour mission from SIN-SYD), then it is usually more economical to send a 773 than a 77W assuming configuration is identical. In CX's case, its 773's are configured in a more economy heavy configuration than its 77W's and is mostly used for runs through SE Asia, Japan and South Korea - the added weight of the 77W costs more to haul around.
Different planes for different missions - the 773 suited airlines like SQ and CX who were replacing 742's and 743's for their 3-4 hour regional routes - the 77W is a plane designed for 744 type (and obviously longer) missions. The 77W is capable of a lot more but if you were running an airline that had no routes over 8 hours (like SIN-NRT), then I would query whether the 77W is the right aircraft. For CX and SQ who both use the 77W on shorter routes, their 77W's fly 12+ hour flights regularly so the "intelligent mis-use" (as CX calls it) of the 77W isn't a problem.
This has been the case-the A330 has decisively won the airlines where very long range is not required. As the A330 has been improved it has proved to be more economical than any version of the 777. However, the A330 does not have the range of the 77W, and so where extremely long range is needed there is no substitute for the 77W. As others have noted, having the long range capability has its penalties in that even when you are not carrying all of that fuel you are still carrying the structure it requires. This is why both the A330 and the 77W are selling well (at least until the 787 becomes readily available, at which point A330 sales will dry up) and the 77E is not selling.
