I have no idea but perhaps there are also some issues with the SG/CA flying permissions. Remember SQ pulled out of YVR because they were only allowed 3 days per week and had to stop in SEL. So - for AC to start with service to SIN, probably there needs to be flying rights first and then analysis of whether or not you could fill the plane. Looking at the Map - YVR-SIN does not provide much of a better stopping point for onward connections than Japan, Hong Kong or even one of the Chinese cities. You are basically over-flying all the other points you could connect to so you are making it longer. Only Indonesia and perhaps Western Australia - or maybe some of the Islands in the Indian ocean are closer with connection in SIN. My thoughts are that SIN is not on the radar for AC too much.