My theory would be the Mainland Chinese carriers on the west coast, or the lack thereof.

From JFK, CX/NH/JL/KE all have good coverage via their respective hubs, let alone the services by ME3. The only missing ones are CA/MU perhaps by 1x daily freq. Even PR have established MNL nonstops.

From LAX/SFO/YVR/ORD, the mainland Chinese carriers are no longer what they used to be. CA/MU/CZ have significantly reduced their services. That gives BR/CI a leg up, either for pax connecting from SE Asia or from mainland China - since the supply is lower now.