Er, there are huge network effects and lock-in. Maintenance and type certification mean that pilots and mechanics get locked into your type. That in turn means

Er, there are huge network effects and lock-in. Maintenance and type certification mean that pilots and mechanics get locked into your type. That in turn means that airlines tend to try to buy from as few manufacturers as possible, so as they grow they're more likely to buy from the manufacturer they bought from last time. There are of course self-sufficiency reasons too. If the West put an embargo in place against China for airliners, they'd be SOL. This is why they're trying to hard to make their own planes (and perhaps more importantly, engines).