Except that this movie about the Great Wall is a fantasy movie and the main character isn't supposed to be Chinese. They're some kind of mercenary types. What's really sad is, if this was to be depicted during the time and these white mercenaries came from somewhere, you'd think they'd have an English accent instead of a North American one.
Someone brought up a good point in a post I read. These companies want to make money, so they need to put an actor\actress into the part who will bring in the most amount of money for them. If you made an Asian the main character, you won't bring in as much money as you would with Matt Damon. Even if Jackie Chan were younger and they used him, Matt would still be the better choice because of acting ability and draw. Until more actors from other cultures start becoming bigger draws, it will continue to be this way.