The weakness of *A is part of UA's problem at LAX.
AA gets a huge shot in the arm every morning, as Qantas deposits 1,000+ pax at LAX - many of whom are going beyond, mostly on AA metal. BA also feeds AA, for service to smaller cities (although BA's services to SAN, LAS, PHX, etc. take away many premium cabin connections). DL gets feed from Korean, and its Chinese partners.
At *A, though, ANA has service to enough other major US markets that the HVF is more likely on a nonstop ex-NRT, while SQ doesn't even put its code on most UA flights ex-hubs (just starting to happen in IAH, IIRC). NZ contributes feed, but then turns around and runs its own LAX-LHR to further cut in to *A options.