A few reasons, that can make direct more expensive operationally & lower demand, respectively:
1) Longer voyages use less fuel as a multi-leg because aircraft don't have to load as much to make the full flight direct. See: "intermediate stop operations". Those 15h and 18h flights are really bad for fuel efficiency, and they can't carry as much cargo. That's a reason Asia<->USA cargo flights frequently stop in Anchorage Alaska. They're also more complex for staff (can only fly so many hours at a time, need to carry relief pilot(s) after X hours)
2) Sometimes transiting a particular country is difficult, so that routing would have lower demand. Sometimes I fly from Paris-USA-Toronto, and that incurs extra headaches (visas, ESTA, luggage restrictions (e.g. food)) that many avoid.
1) Longer voyages use less fuel as a multi-leg because aircraft don't have to load as much to make the full flight direct. See: "intermediate stop operations". Those 15h and 18h flights are really bad for fuel efficiency, and they can't carry as much cargo. That's a reason Asia<->USA cargo flights frequently stop in Anchorage Alaska. They're also more complex for staff (can only fly so many hours at a time, need to carry relief pilot(s) after X hours)
2) Sometimes transiting a particular country is difficult, so that routing would have lower demand. Sometimes I fly from Paris-USA-Toronto, and that incurs extra headaches (visas, ESTA, luggage restrictions (e.g. food)) that many avoid.