Shortage of 747 spare parts and maintenance resources at certain airports could be one of the factors to switch to 77W on the SFO-TPE route.
Frequnent 747s services are still seen at PEK, ICN, LHR and FRA and these airports may have enough spare parts and mechanics to work on the 747 if needed.
TPE (and PVG) may not have the adequate 747 support on the ground.