Yes CX did have a Day time flight out of LAX 3 yrs ago. However due to low yield and inconvenient connection schedules, they've decided to make both flights depart during the night hours since the majority of the LAX pax have connecting flight beyond HKG. Night time flights with morning arrivals make better connections. As for the YVR daytime, like you said it, most are HK residents thus HKG is the final destination, so a night time arrival into HKG is no big deal.