For whatever reason can be thought up, it clearly is supply/demand. We all know the China has become an industrial powerhouse latley. Supply of carriers that have both the rights to, and the equipment to fly between the two countries has not grown as fast as the demand for travel has. There are significant barriers to entry for new competitors, both regulated by the g-men, and in terms of cost/feeder network. The plane required to fly from most cities to get to China is rather large and expensive. Multiple planes are required for any 1 route given the length of flight. Origin/destination traffic may not exist in many markets where new possible competition exists, without first developing a domestic route network capable of supplying connecting passengers to make such large/costly (to buy and operate) investment easy. Then you must apply for one of the limited slots, and wait. You must show that you can provided the best service, and can benefit the US economy better than your competition. ETOPS and 747 are the 2 commercial planes capable of doing this. Neither are cheap to maintain. Fuel costs are high, (not that fares aren't) which makes a startup less likely to be profitable (maybe indy-air would have lasted longer and eventaully worked out had they had low fuel prices.)

To wrap it up, demand>supply.