My guess is a combination of yield and traffic. The internet bubble collapse has seriously cut F/J traffic out of SFO, a huge reduction from 3 years ago (at least for the flights that I've taken from SFO in F). CX obviously finds more money out of YVR and YYZ than SFO. I've been offered some very low consolidator fares out of SFO. Presumably these will end when the planes are converted to NBC (fewer seats and a better product). CX does have quite a few AA codeshares into SFO, which suggests they would like to expand this service. But the CX lounge is so bad that I stay at the DL Crown room until it closes (at midnight). Pretty sad for a brand new terminal that cost hundreds of millions of dollars to construct (but UA seems to have made sure that the other airlines didn't get decent lounges there).