Beijing doesn't like their carriers competing directly with one another, so not everyone is allowed onto the Shanghai and Beijing route
Pudong(Shanghai) is being serviced by MU, CA and QF. Once the bilateral air agreements are established it becomes the domain of the airport operator as to who can operate there. Shanghai is not a large hub for AC, PVG is a base for MU. I don't think Beijing will restrict who operates into these ports now with the explosion of new Chinese carriers.