As this question has been asked several times in the forum, I never reacted to that question, so here's my opinion. Like many members here already said, it has to do with different markets requiring different aircraft capacities. As the A343 and 777 come very close to each other in terms of range/capacity they do serve different purposes. While CX
uses their A343's on Asian routes, they are also used on international flights such as Amsterdam and Canadian destinations, while the 777's of CX
are only used on Asian/Australian routes. As for Singapore Airlines, they have sold all their A343 aircraft and all these aircraft have been placed with other airlines AFAIK. Their A345 fleet is only used on their new ultra-long range destinations which cannot be flown by the current 777's, until the 777LR will be available. As for Air France and China Southern concerned, I think politics play part in this. France being the main "shareholder" within EADS and China (CAAC) which orders its aircraft in a single order and later divides them over all the Chinese airlines. As for Emirates I don't know the reason exactly, but I can imagine politics also play a role there, but I'm not sure, so don't flame me if I'm wrong.
This is only my opinion, any additional comments are welcomed.