Because this is a low yield tourist route and AC can better and more profitably deploy its aircraft to other cities in the region that have a higher percentage of business travelers, and thus are more profitable. Not to mention the whole point of an Alliance like STAR is to have regional carriers who can pick up your transfer passengers and get them to hundreds rather than a few destinations within the region. Not to mention it has only been in the past few years that nonstop flights are possible to this part of the world, so AC would have to do what UA and NW do at NRT and HKG and create a mini-hub with several flights interchanging passengers to keep the planes full to destinations like BKK and SIN. (CP used to fly to BKK with a stop at NRT, but volumes never made this a profitable turnaround.)