I think the reason why CX doesn't have double daily service from SFO is because there are already three airlines that have daily service already between SFO and HKG. And all three airlines use 747s.

I don't think any other airline besides CX flies between LAX and HKG, unless you count cargo flights.

Speaking of which, last night I was walking outside my home near ORD and I saw a CX 747 making a descent. I almost had a heart attack until I realized it was probably for cargo. What a letdown...