As DUF mentioned, crewing is a major issue - SYD-LAX-SYD is (IIRC) a four day trip for cabin crew - adding in a LAX-JFK-LAX turnaround and it becomes a seven day trip (I am sure one of our CC members will elaborate on this).
So instead of needing four crews to operate a service, you need seven (obviously a lot more than this, but you get the picture).
This is why the previously mooted tags to DFW and ORD have been canned, the costs associated with it would greatly exceed to revenue.
Dave