R-nut,
crew-commonality has a big part to do with it; the 777 has no shared types (I think the 767-400 could be, though). The A-330 is also a lot cheaper. It's also not as capable as the 777 IGW version, but I think that's a fallacious comparison. I don't know of any carrier flying A-330's on 14hr-plus legs with a full load....that's what the A-340-600 is for. But the 777 does it every day on 2 engines, and hence is in a different market.
I think...let's let the Cathay chaps comment here. They've the most experience of operating and comparing both types (all 3, actually).